iShares U.S. Aggregate Bond Index Fund

Fund Overview

Key Facts About iShares U.S. Aggregate Bond Index Fund

Investment Objective

iShares U.S. Aggregate Bond Index Fund (the “Fund”), a series of BlackRock Funds III (the “Trust”), seeks to provide investment results that correspond to the total return performance of fixed-income securities in the aggregate, as represented by the Bloomberg U.S. Aggregate Bond Index (the “Bloomberg U.S. Aggregate Index” or the “Underlying Index”).

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy, hold and sell shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock Fund Advisors (“BFA”) and its affiliates) (each, a “Financial Intermediary”), which are not reflected in the table and example below.

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)		Investor A Shares		Institutional Shares
Management Fee1,2,3		0.04%		0.04%
Distribution and/or Service (12b-1) Fees		0.25%		None
Other Expenses4		0.06%		0.06%
Administration Fees	0.06%		0.06%
Independent Expenses4	—		—
Total Annual Fund Operating Expenses		0.35%		0.10%
Fee Waivers and/or Expense Reimbursements2,3,4		—		—
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements2,3,4		0.35%		0.10%

The fees and expenses shown in the table above and the example that follows include both the expenses of the Fund and the Fund’s share of the allocated expenses of U.S. Total Bond Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio (“MIP”). Management fees are paid by the Master Portfolio.

BFA, the investment adviser for the Master Portfolio, has contractually agreed to waive its management fee by the amount of investment advisory fees the Master Portfolio pays to BFA indirectly through its investment in affiliated money market funds through June 30, 2025. The agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of MIP or by a vote of a majority of the outstanding voting securities of the Master Portfolio.

As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 34, BFA has contractually agreed to waive the management fee with respect to any portion of the Master Portfolio’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BFA or its affiliates that have a contractual management fee, through June 30, 2025. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of MIP or by a vote of a majority of the outstanding voting securities of the Master Portfolio.

Independent Expenses consist of the Fund’s allocable portion of the fees and expenses of the independent trustees of the Trust and MIP, counsel to such independent trustees and the independent registered public accounting firm that provides audit services to the Fund and the Master Portfolio. BlackRock Advisors, LLC (“BAL”), the administrator for the Fund, and BFA have contractually agreed to reimburse, or provide offsetting credits to, the Fund and the Master Portfolio, as applicable, for Independent Expenses through June 30, 2025. After giving effect to such contractual arrangements, Independent Expenses will be 0.00%. Such contractual arrangements may not be terminated prior to July 1, 2025 without the consent of the Boards of Trustees of the Trust and of MIP.

Example:

This Example is intended to help you compare the cost of investing in shares of the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in shares of the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 Year	3 Years	5 Years	10 Years
Investor A Shares	$36	$113	$197	$443
Institutional Shares	$10	$32	$56	$128

Portfolio Turnover:

The Master Portfolio pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Master Portfolio’s portfolio turnover rate was 78% of the average value of its portfolio.

Principal Investment Strategies of the Fund

The Fund pursues its investment objective by seeking to match the total return performance of the Bloomberg U.S. Aggregate Index, which is composed of approximately 10,000 fixed-income securities. The fixed-income securities that comprise the Bloomberg U.S. Aggregate Index include U.S. Government securities and corporate bonds, as well as mortgage-backed securities, asset-backed securities and commercial mortgage-backed securities. All securities in the Bloomberg U.S. Aggregate Index are investment-grade. The Fund maintains a weighted average maturity consistent with that of the Bloomberg U.S. Aggregate Index, which generally ranges between 5 and 10 years. The Fund invests in a representative sample of these securities. Securities are selected for investment by the Fund in accordance with their relative proportion within the Bloomberg U.S. Aggregate Index as well as based on credit quality, issuer sector, maturity structure, coupon rates and callability, among other factors. BFA, the investment adviser to the Master Portfolio in which the Fund invests, considers investments that provide substantially similar exposure to securities in the Bloomberg U.S. Aggregate Index to be investments comprising the Fund’s benchmark index. For example, the Fund may invest in mortgage dollar rolls and participate in to-be-announced (“TBA”) transactions on a regular basis to obtain exposure to mortgage-backed securities.

The Fund is managed by determining which securities are to be purchased or sold to reflect, to the extent feasible, the investment characteristics of its benchmark index. Under normal circumstances, at least 90% of the value of the Fund’s assets, plus the amount of any borrowing for investment purposes, is invested in securities comprising the Bloomberg U.S. Aggregate Index, which, for the Fund, are considered bonds.

The Fund is a “feeder” fund that invests all of its assets in the Master Portfolio of MIP, which has the same investment objective and strategies as the Fund. All investments are made at the Master Portfolio level. This structure is sometimes called a “master/feeder” structure. The Fund’s investment results will correspond directly to the investment results of the Master Portfolio. For simplicity, the prospectus uses the name of the Fund or the term “Fund” (as applicable) to include the Master Portfolio.

Principal Risks of Investing in the Fund

Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. The following is a summary description of the principal risks of investing in the Fund. The relative significance of each risk factor below may change over time and you should review each risk factor carefully.

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Debt Securities Risk — Debt securities, such as bonds, involve risks, such as credit risk, interest rate risk, extension risk, and prepayment risk, each of which are described in further detail below:

Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation.

Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise.

measure of the price sensitivity of a debt security or portfolio of debt securities to relative changes in interest rates.) The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management.

To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities.

These basic principles of bond prices also apply to U.S. Government securities. A security backed by the “full faith and credit” of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change.

A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund’s performance.

Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.

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Index-Related Risk — There is no guarantee that the Fund’s investment results will have a high degree of correlation to those of the Underlying Index or that the Fund will achieve its investment objective. Market disruptions or high volatility, other unusual market circumstances and regulatory restrictions could have an adverse effect on the Fund’s ability to adjust its exposure to the required levels in order to track the Underlying Index. Errors in index data, index computations or the construction of the Underlying Index in accordance with its methodology may occur from time to time and may not be identified and corrected by the index provider for a period of time or at all, which may have an adverse impact on the Fund and its shareholders. Unusual market conditions or other unforeseen circumstances (such as natural disasters, political unrest or war) may impact the index provider or a third-party data provider, and could cause the index provider to postpone a scheduled rebalance. This could cause the Underlying Index to vary from its normal or expected composition.

An index fund has operating and other expenses while an index does not. As a result, while the Fund will attempt to track the Underlying Index as closely as possible, it will tend to underperform the Underlying Index to some degree over time. If an index fund is properly correlated to its stated index, the fund will perform poorly when the index performs poorly.

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Passive Investment Risk — Because BFA does not select individual companies in the index that the Fund tracks, the Fund may hold securities of companies that present risks that an investment adviser researching individual securities might seek to avoid.

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Tracking Error Risk — The Fund may be subject to tracking error, which is the divergence of the Fund’s performance from that of the Underlying Index. Tracking error may occur because of differences between the securities and other instruments held in the Fund’s portfolio and those included in the Underlying Index, pricing differences (including, as applicable, differences between a security’s price at the local market close and the Fund’s valuation of a security at the time of calculation of the Fund’s net asset value (“NAV”)), differences in transaction costs, the Fund’s holding of uninvested cash, differences in timing of the accrual of or the valuation of dividends or other distributions, interest, the requirements to maintain pass-through tax treatment, portfolio transactions carried out to minimize the distribution of capital gains to shareholders, changes to the Underlying Index and the cost to the Fund of complying with various new or existing regulatory requirements, among other reasons. These risks may be heightened during times of increased market volatility or other unusual market conditions. In addition, tracking error may result because the Fund incurs fees and expenses, while the Underlying Index does not.

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Dollar Rolls Risk — Dollar rolls involve the risk that the market value of the securities that the Fund is committed to buy may decline below the price of the securities the Fund has sold. These transactions may involve leverage.

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High Portfolio Turnover Risk — The Fund may engage in active and frequent trading of its portfolio securities. High portfolio turnover (more than 100%) may result in increased transaction costs to the Fund, including brokerage commissions, dealer mark-ups and other transaction costs on the sale of the securities and on reinvestment in other securities. The sale of Fund portfolio securities may result in the realization and/or distribution to shareholders of higher capital gains or losses as compared to a fund with less active trading policies. These effects of higher than normal portfolio turnover may adversely affect Fund performance. In addition, investment in mortgage dollar rolls and participation in TBA transactions may significantly increase the Fund’s portfolio turnover rate. A TBA transaction is a method of trading mortgage-backed securities where the buyer and seller agree upon general trade parameters such as agency, settlement date, par amount, and price at the time the contract is entered into but the mortgage-backed securities are delivered in the future, generally 30 days later.

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Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. The value of a security or other asset may decline due to changes in general market conditions, economic trends or events that are not specifically related to the issuer of the security or other asset, or factors that affect a particular issuer or issuers, exchange, country, group of countries, region, market, industry, group of industries, sector or asset class. Local, regional or global events such as war, acts of terrorism, the spread of infectious illness or other public health issues like pandemics or epidemics, recessions, or other events could have a significant impact on the Fund and its investments. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

An outbreak of an infectious coronavirus (COVID-19) that was first detected in December 2019 developed into a global pandemic that has resulted in numerous disruptions in the market and has had significant economic impact leaving general concern and uncertainty. Although vaccines have been developed and approved for use by various governments, the duration of the pandemic and its effects cannot be predicted with certainty. The impact of this coronavirus, and other epidemics and pandemics that may arise in the future, could affect the economies of many nations, individual companies and the market in general ways that cannot necessarily be foreseen at the present time.

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Mortgage- and Asset-Backed Securities Risks — Mortgage- and asset-backed securities represent interests in “pools” of mortgages or other assets, including consumer loans or receivables held in trust. Mortgage- and asset-backed securities are subject to credit, interest rate, prepayment and extension risks. These securities also are subject to risk of default on the underlying mortgage or asset, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain mortgage-backed securities.

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Risk of Investing in the United States — Certain changes in the U.S. economy, such as when the U.S. economy weakens or when its financial markets decline, may have an adverse effect on the securities to which the Fund has exposure.

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U.S. Government Obligations Risk — Certain securities in which the Fund may invest, including securities issued by certain U.S. Government agencies and U.S. Government sponsored enterprises, are not guaranteed by the U.S. Government or supported by the full faith and credit of the United States. In addition, circumstances could arise that could prevent the timely payment of interest or principal on U.S. Government obligations, such as reaching the legislative “debt ceiling.” Such non-payment could result in losses to the Fund and substantial negative consequences for the U.S. economy and the global financial system.

Performance Information

The information shows how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The average annual total returns table (before and after taxes) compares the Fund’s performance to that of the Bloomberg U.S. Aggregate Index. To the extent that dividends and distributions have been paid by the Fund, the performance information of the Fund in the chart and table assumes reinvestment of the dividends and distributions. As with all such investments, past performance (before and after taxes) is not necessarily an indication of future results. The table includes all applicable fees. If BFA, BAL and their affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com or can be obtained by phone at (800) 882-0052.

Investor A Shares
ANNUAL TOTAL RETURNS
iShares U.S. Aggregate Bond Index Fund
As of 12/31

During the ten-year period shown in the bar chart, the highest return for a quarter was 6.53% (quarter ended December 31, 2023) and the lowest return for a quarter was -5.90% (quarter ended March 31, 2022).

For the periods ended 12/31/23 Average Annual Total Returns	1 Year	5 Years	10 Years
iShares U.S. Aggregate Bond Index Fund — Investor A Shares
Return Before Taxes	5.39 %	0.80 %	1.44 %
Return After Taxes on Distributions	4.15 %	(0.16 )%	0.51 %
Return After Taxes on Distributions and Sale of Fund Shares	3.17 %	0.23 %	0.71 %
iShares U.S. Aggregate Bond Index Fund — Institutional Shares
Return Before Taxes	5.66 %	1.03 %	1.69 %
Bloomberg U.S. Aggregate Bond Index (Reflects no deduction for fees, expenses or taxes)	5.53 %	1.10 %	1.81 %

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns are shown for Investor A Shares only, and the after-tax returns for Institutional Shares will vary.

Investment Adviser

The Master Portfolio’s investment adviser is BlackRock Fund Advisors (previously defined as “BFA”).

Portfolio Managers

Portfolio Manager	Portfolio Manager of the Master Portfolio Since	Title
James Mauro	2011	Managing Director of BlackRock, Inc.
Karen Uyehara	2020	Managing Director of BlackRock, Inc.

Purchase and Sale of Fund Shares

You may purchase or redeem shares of the Fund each day the New York Stock Exchange is open. To purchase or sell shares you should contact your Financial Intermediary, or, if you hold your shares through the Fund, you should contact the Fund by phone at (800) 441-7762, by mail (c/o BlackRock Funds III, P.O. Box 534429, Pittsburgh, Pennsylvania 15253-4429), or by the Internet at www.blackrock.com. The Fund’s initial and subsequent investment minimums generally are as follows, although the Fund may reduce or waive the minimums in some cases:

	Investor A Shares	Institutional Shares
Minimum Initial Investment	$1,000 for all accounts except: •$50, if establishing an Automatic Investment Plan. •There is no investment minimum for employer- sponsored retirement plans (not including SEP IRAs, SIMPLE IRAs or SARSEPs). •There is no investment minimum for certain fee- based programs.	There is no minimum initial investment for: •Employer-sponsored retirement plans (not including SEP IRAs, SIMPLE IRAs or SARSEPs), state sponsored 529 college savings plans, collective trust funds, investment companies or other pooled investment vehicles, unaffiliated thrifts and unaffiliated banks and trust companies, each of which may purchase shares of the Fund through a Financial Intermediary that has entered into an agreement with the Fund’s distributor to purchase such shares. •Clients of Financial Intermediaries that: (i) charge such clients a fee for advisory, investment consulting, or similar services or (ii) have entered into an agreement with the Fund’s distributor to offer Institutional Shares through a no-load program or investment platform. •Clients investing through a self-directed IRA brokerage account program sponsored by a retirement plan record-keeper, provided that such program offers only mutual fund options and that the program maintains an account with the Fund on an omnibus basis. $2 million for individuals and “Institutional Investors,” which include, but are not limited to, endowments, foundations, family offices, local, city, and state governmental institutions, corporations and insurance company separate accounts who may purchase shares of the Fund through a Financial Intermediary that has entered into an agreement with the Fund’s distributor to purchase such shares. $1,000 for: •Clients investing through Financial Intermediaries that offer such shares on a platform that charges a transaction based sales commission outside of the Fund. •Tax-qualified accounts for insurance agents that are registered representatives of an insurance company’s broker-dealer that has entered into an agreement with the Fund’s distributor to offer Institutional Shares, and the family members of such persons.
Minimum Additional Investment	$50 for all accounts (with the exception of certain employer-sponsored retirement plans which may have a lower minimum).	No subsequent minimum.

Tax Information

The Fund’s dividends and distributions may be subject to U.S. federal income taxes and may be taxed as ordinary income or capital gains, unless you are a tax-exempt investor or are investing through a qualified tax-exempt plan described in section 401(a) of the Internal Revenue Code of 1986, as amended, in which case you may be subject to U.S. federal income tax when distributions are received from such tax-deferred arrangements.

Payments to Broker/Dealers and Other Financial Intermediaries

If you purchase shares of the Fund through a Financial Intermediary, the Fund and BlackRock Investments, LLC, the Fund’s distributor, or its affiliates may pay the Financial Intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the Financial Intermediary and your individual financial professional to recommend the Fund over another investment.

Details About the Fund

Included in this prospectus are sections that tell you about buying and selling shares, management information, shareholder features of iShares U.S. Aggregate Bond Index Fund (the “Fund”), a series of BlackRock Funds III (the “Trust”), and your rights as a shareholder.

The Fund is a “feeder fund” that invests all of its assets in U.S. Total Bond Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio (“MIP”), that has the same investment objective and strategies as the Fund. All investments will be made at the Master Portfolio level. This structure is sometimes called a “master/feeder” structure. The Fund’s investment results will correspond directly to the investment results of the Master Portfolio. For simplicity, this prospectus uses the name of the Fund or the term “Fund” to include, where applicable, the Master Portfolio.

Details About the Fund’s Investment Strategies

Investment Objective

The Fund seeks to provide investment results that correspond to the total return performance of fixed-income securities in the aggregate, as represented by the Bloomberg U.S. Aggregate Bond Index (the “Bloomberg U.S. Aggregate Index” or the “Underlying Index”). The Fund’s investment objective may be changed by the Trust’s Board of Trustees without shareholder approval.

Investment Strategies

Under normal circumstances, at least 90% of the value of the Fund’s assets, plus the amount of any borrowing for investment purposes, is invested in securities comprising the Bloomberg U.S. Aggregate Index, which, for the Fund, are considered bonds. The Fund will provide shareholders with at least 60 days’ notice of any change to this policy. The Fund attempts to achieve, in both rising and falling markets, a correlation of at least 95% between the total return of its net assets before fees and expenses and the total return of the Fund’s benchmark index, the Bloomberg U.S. Aggregate Index. Notwithstanding the factors described below, perfect (100%) correlation would be achieved if the total return of the Fund’s net assets, before fees and expenses, increased or decreased exactly as the total return of the Fund’s benchmark index increased or decreased. The Fund’s ability to match its investment performance to the investment performance of its benchmark index may be affected by, among other things, the Fund’s expenses, the amount of cash and cash equivalents held by the Fund, the manner in which the total return of the Fund’s benchmark index is calculated, the size of the Fund’s investment portfolio, and the timing, frequency and size of shareholder purchases and redemptions.

The Fund utilizes sampling techniques that are designed to allow the Fund to duplicate substantially the investment performance of the Bloomberg U.S. Aggregate Index. However, the Fund is not expected to track the Bloomberg U.S. Aggregate Index with the same degree of accuracy that complete replication of the Bloomberg U.S. Aggregate Index would provide. No attempt is made to manage the Fund using economic, financial or market analysis. In addition, at times, the portfolio composition of the Fund may be altered (or “rebalanced”) to reflect changes in the characteristics of the index that the Fund tracks. The Fund may invest in mortgage dollar rolls and participate in to-be-announced (“TBA”) transactions on a regular basis to obtain exposure to mortgage-backed securities.

The Fund also may engage in futures and options transactions and other derivative securities transactions and lend its portfolio securities, each of which involves risk. The Fund may use futures contracts, options and other derivative transactions to manage its short-term liquidity and/or as substitutes for comparable market positions in the securities in its benchmark index. The Fund may also invest in high-quality money market instruments, including shares of money market funds advised by BlackRock Fund Advisors (“BFA”) or its affiliates.

Investors look to indexes as a standard of market performance. Indexes are model portfolios, that is, groups of stocks or bonds selected to represent an entire market or market segment. One way an index fund seeks to match an index’s performance, before fees and expenses, is by buying and selling all of the index’s securities in the same proportion as they are reflected in the index.

There were approximately 10,000 fixed-income securities included in the Bloomberg U.S. Aggregate Index. The Fund maintains a weighted average maturity consistent with that of the Bloomberg U.S. Aggregate Index, which generally ranges between 5 and 10 years. As a practical matter, it would be inefficient for the Fund to hold each security

included in the Bloomberg U.S. Aggregate Index. The Fund can, however, substantially replicate the Bloomberg U.S. Aggregate Index’s profile by holding a representative sample of securities in the Bloomberg U.S. Aggregate Index. It may, for example, hold U.S. Government obligations and corporate bonds in a similar proportion to the Bloomberg U.S. Aggregate Index. Additionally, it can match certain Bloomberg U.S. Aggregate Index features such as:

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Average time to maturity for both government and corporate securities;

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Securities’ coupon rates, which are the interest rates securities pay based on their face values;

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Economic sectors represented by securities;

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Credit quality of securities (all securities in the Bloomberg U.S. Aggregate Index are rated investment-grade); and

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Whether or not securities are callable, which means the issuer has the right to repay principal and interest before maturity.

The Fund is designed for investors who desire a convenient way to invest in bonds issued in the United States. Although this market has increased in value over the long term, it fluctuates and has also decreased in value over shorter time periods.

The Fund does not by itself constitute a balanced investment program. Diversifying your investments by buying shares in other funds may improve your long-term return as well as reduce volatility.

The Bloomberg U.S. Aggregate Index is maintained by Bloomberg Index Services Limited (the “Index Provider”). The Index Provider does not sponsor, endorse, sell or promote the Fund or the Master Portfolio. The Index Provider makes no representation or warranty, expressed or implied, regarding the advisability of investing in the Fund or its Master Portfolio. Neither BFA nor BlackRock Advisors, LLC (“BAL”) has or will have a role in maintaining the Bloomberg U.S. Aggregate Index.

The past performance of the Underlying Index is not a guide to future performance. BFA does not guarantee the accuracy or the completeness of the Underlying Index or any data included therein and BFA shall have no liability for any errors, omissions or interruptions therein. BFA makes no warranty, express or implied, to the owners of shares of the Fund or to any other person or entity, as to results to be obtained by the Fund from the use of the Underlying Index or any data included therein. Without limiting any of the foregoing, in no event shall BFA have any liability for any special, punitive, direct, indirect or consequential damages (including lost profits), even if notified of the possibility of such damages.

ABOUT THE PORTFOLIO MANAGEMENT TEAM OF THE FUND/MASTER PORTFOLIO

The Master Portfolio is managed by a team of financial professionals. James Mauro and Karen Uyehara are the portfolio managers and are jointly and primarily responsible for the day-to-day management of the Master Portfolio. Please see “Management of the Fund — Portfolio Managers” for additional information about the portfolio management team.

Investment Risks

This section contains a discussion of the general risks of investing in the Fund. The “Investment Objective and Policies” section in the Fund’s Statement of Additional Information (the “SAI”) also includes more information about the Fund, its investments and the related risks. As with any fund, there can be no guarantee that the Fund will meet its investment objective or that the Fund’s performance will be positive for any period of time. An investment in the Fund is not a deposit in any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or by any bank or governmental agency. The order of the below risk factors does not indicate the significance of any particular risk factor.

Principal Risks of Investing in the Fund

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The Fund may be subject to a greater risk of rising interest rates due to the recent period of historically low interest rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund’s investments would be expected to decrease by 10%. (Duration is a measure of the price sensitivity of a debt security or portfolio of debt securities to relative changes in interest rates.) The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management.

The Federal Reserve has recently begun to raise the federal funds rate as part of its efforts to address rising inflation. There is a risk that interest rates will continue to rise, which will likely drive down the prices of bonds and other fixed-income securities. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from mutual funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund’s performance.

During periods of very low or negative interest rates, the Fund may be unable to maintain positive returns. Certain countries have recently experienced negative interest rates on certain fixed-income instruments. Very low or negative interest rates may magnify interest rate risk. Changing interest rates, including rates that fall below zero, may have unpredictable effects on markets, may result in heightened market volatility and may detract from Fund performance to the extent the Fund is exposed to such interest rates.

Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Rising interest rates tend to extend the duration of securities, making them more sensitive to changes in interest rates. The value of longer-term securities generally changes more in response to changes in interest rates than shorter-term securities. As a result, in a period of rising interest rates, securities may exhibit additional volatility and may lose value.

Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. In periods of falling interest rates, the rate of prepayments tends to increase (as does price fluctuation) as borrowers are motivated to pay off debt and refinance at new lower rates. During such periods, reinvestment of the prepayment proceeds by the management team will generally be at lower rates of return than the return on the assets that were prepaid. Prepayment reduces the yield to maturity and the average life of the security.

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Dollar Rolls Risk — A dollar roll transaction involves a sale by the Fund of a mortgage-backed, U.S. Treasury or other security (as permitted by the Fund’s investment strategies) concurrently with an agreement by the Fund to repurchase a similar security at a later date at an agreed-upon price. The market value of the securities the Fund is required to purchase may decline below the agreed upon repurchase price of those securities. If the broker/dealer to whom the Fund sells securities becomes insolvent, the Fund’s right to purchase or repurchase securities may be restricted. Successful use of dollar rolls may depend upon the adviser’s ability to correctly predict interest rates and prepayments, depending on the underlying security. There is no assurance that dollar rolls can be successfully employed.

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other securities. The sale of Fund portfolio securities may result in the realization and/or distribution to shareholders of higher capital gains or losses as compared to a fund with less active trading policies. These effects of higher than normal portfolio turnover may adversely affect Fund performance. In addition, investment in mortgage dollar rolls and participation in TBA transactions may significantly increase the Fund’s portfolio turnover rate. A TBA transaction is a method of trading mortgage-backed securities where the buyer and seller agree upon general trade parameters such as agency, settlement date, par amount and price at the time the contract is entered into but the mortgage-backed securities are delivered in the future, generally 30 days later.

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Index-Related Risk — The Fund seeks to achieve a return that corresponds generally to the price and yield performance, before fees and expenses, of the Underlying Index as published by the Index Provider. There is no assurance that the Index Provider or any agents that may act on its behalf will compile the Underlying Index accurately, or that the Underlying Index will be determined, composed or calculated accurately. While the Index Provider provides descriptions of what the Underlying Index is designed to achieve, neither the Index Provider nor its agents provide any warranty or accept any liability in relation to the quality, accuracy or completeness of the Underlying Index or its related data, and they do not guarantee that the Underlying Index will be in line with the Index Provider’s methodology. BFA’s mandate as described in this prospectus is to manage the Fund consistently with the Underlying Index provided by the Index Provider to BFA. BFA does not provide any warranty or guarantee against the Index Provider’s or any agent’s errors. Errors in respect of the quality, accuracy and completeness of the data used to compile the Underlying Index may occur from time to time and may not be identified and corrected by the Index Provider for a period of time or at all, particularly where the indices are less commonly used as benchmarks by funds or managers. Such errors may negatively or positively impact the Fund and its shareholders. For example, during a period where the Underlying Index contains incorrect constituents, the Fund would have market exposure to such constituents and would be underexposed to the Underlying Index’s other constituents. Shareholders should understand that any gains from Index Provider errors will be kept by the Fund and its shareholders and any losses or costs resulting from Index Provider errors will be borne by the Fund and its shareholders.

Unusual market conditions or other unforeseen circumstances (such as natural disasters, political unrest or war) may impact the Index Provider or a third-party data provider, and could cause the Index Provider to postpone a scheduled rebalance to the Underlying Index. This could cause the Underlying Index to vary from its normal or expected composition. The postponement of a scheduled rebalance could mean that constituents of the Underlying Index that would otherwise be removed at rebalance due to changes in market capitalizations, issuer credit ratings, or other reasons may remain, causing the performance and constituents of the Underlying Index to vary from those expected under normal conditions. Apart from scheduled rebalances, the Index Provider or its agents may carry out additional ad hoc rebalances to the Underlying Index due to reaching certain weighting constraints, unusual market conditions or corporate events or, for example, to correct an error in the selection of index constituents. When the Underlying Index is rebalanced and the Fund in turn rebalances its portfolio to attempt to increase the correlation between the Fund’s portfolio and the Underlying Index, any transaction costs and market exposure arising from such portfolio rebalancing will be borne directly by the Fund and its shareholders. Therefore, errors and additional ad hoc rebalances carried out by the Index Provider or its agents to the Underlying Index may increase the costs to and the tracking error risk of the Fund.

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coronavirus, and other epidemics and pandemics that may arise in the future, could affect the economies of many nations, individual companies and the market in general ways that cannot necessarily be foreseen at the present time.

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Mortgage- and Asset-Backed Securities Risks — Mortgage-backed securities (residential and commercial) and asset-backed securities represent interests in “pools” of mortgages or other assets, including consumer loans or receivables held in trust. Although asset-backed and commercial mortgage-backed securities (“CMBS”) generally experience less prepayment than residential mortgage-backed securities, mortgage-backed and asset-backed securities, like traditional fixed-income securities, are subject to credit, interest rate, prepayment and extension risks.

Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain mortgage-backed securities. The Fund’s investments in asset-backed securities are subject to risks similar to those associated with mortgage-related securities, as well as additional risks associated with the nature of the assets and the servicing of those assets. These securities also are subject to the risk of default on the underlying mortgages or assets, particularly during periods of economic downturn. Certain CMBS are issued in several classes with different levels of yield and credit protection. The Fund’s investments in CMBS with several classes may be in the lower classes that have greater risks than the higher classes, including greater interest rate, credit and prepayment risks.

Mortgage-backed securities may be either pass-through securities or collateralized mortgage obligations (“CMOs”). Pass-through securities represent a right to receive principal and interest payments collected on a pool of mortgages, which are passed through to security holders. CMOs are created by dividing the principal and interest payments collected on a pool of mortgages into several revenue streams (“tranches”) with different priority rights to portions of the underlying mortgage payments. Certain CMO tranches may represent a right to receive interest only (“IOs”), principal only (“POs”) or an amount that remains after floating-rate tranches are paid (an “inverse floater”). These securities are frequently referred to as “mortgage derivatives” and may be extremely sensitive to changes in interest rates. Interest rates on inverse floaters, for example, vary inversely with a short-term floating rate (which may be reset periodically). Interest rates on inverse floaters will decrease when short-term rates increase, and will increase when short-term rates decrease. These securities have the effect of providing a degree of investment leverage. In response to changes in market interest rates or other market conditions, the value of an inverse floater may increase or decrease at a multiple of the increase or decrease in the value of the underlying securities. If the Fund invests in CMO tranches (including CMO tranches issued by government agencies) and interest rates move in a manner not anticipated by Fund management, it is possible that the Fund could lose all or substantially all of its investment. Certain mortgage-backed securities in which the Fund may invest may also provide a degree of investment leverage, which could cause the Fund to lose all or substantially all of its investment.

The mortgage market in the United States has experienced difficulties that may adversely affect the performance and market value of certain of the Fund’s mortgage-related investments. Delinquencies and losses on mortgage loans (including subprime and second-lien mortgage loans) and a decline in or flattening of real estate values (in each case as has been experienced and may continue to be experienced in many housing markets) may exacerbate such delinquencies and losses. Also, a number of mortgage loan originators have experienced serious financial difficulties or bankruptcy. Reduced investor demand for mortgage loans and mortgage-related securities and increased investor yield requirements have caused limited liquidity in the secondary market for mortgage-related securities, which can adversely affect the market value of mortgage-related securities. It is possible that such limited liquidity in such secondary markets could continue or worsen.

Asset-backed securities entail certain risks not presented by mortgage-backed securities, including the risk that in certain states it may be difficult to perfect the liens securing the collateral backing certain asset-backed securities. In addition, certain asset-backed securities are based on loans that are unsecured, which means that there is no collateral to seize if the underlying borrower defaults.

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Risk of Investing in the United States — A decrease in imports or exports, changes in trade regulations, inflation and/or an economic recession in the United States may have a material adverse effect on the U.S. economy and the securities listed on U.S. exchanges. Proposed and adopted policy and legislative changes in the United States are changing many aspects of financial, commercial, public health, environmental, and other regulation and may have a significant effect on U.S. markets generally, as well as on the value of certain securities. Governmental agencies project that the United States will continue to maintain elevated public debt levels for the foreseeable future. Although elevated debt levels do not necessarily indicate or cause economic problems, elevated public debt service costs may constrain future economic growth.

The United States has developed increasingly strained relations with a number of foreign countries. If relations with certain countries deteriorate, it could adversely affect U.S. issuers as well as non-U.S. issuers that rely on the United States for trade. The United States has also experienced increased internal political discord, as well as significant challenges in managing and containing the outbreak of COVID-19. If these trends were to continue, it may have an adverse impact on the U.S. economy and the issuers in which the Fund invests.

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U.S. Government Obligations Risk — Not all U.S. Government securities are backed by the full faith and credit of the United States. Obligations of certain agencies, authorities, instrumentalities and sponsored enterprises of the U.S. Government are backed by the full faith and credit of the United States (e.g., the Government National Mortgage Association); other obligations are backed by the right of the issuer to borrow from the U.S. Treasury (e.g., the Federal Home Loan Banks) and others are supported by the discretionary authority of the U.S. Government to purchase an agency’s obligations. Still others are backed only by the credit of the agency, authority, instrumentality or sponsored enterprise issuing the obligation. No assurance can be given that the U.S. Government would provide financial support to any of these entities if it is not obligated to do so by law. In addition, circumstances could arise that could prevent the timely payment of interest or principal on U.S. Government obligations, such as reaching the legislative “debt ceiling.” Such non-payment could result in losses to the Fund and substantial negative consequences for the U.S. economy and the global financial system.

Other Risks of Investing in the Fund

The Fund may also be subject to certain other non-principal risks associated with its investments and investment strategies, including:

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Concentration Risk — The Fund reserves the right to concentrate its investments (i.e., invest 25% or more of its total assets in securities of issuers in a particular industry) to approximately the same extent that the Underlying Index concentrates in a particular industry. To the extent the Fund concentrates in a particular industry, it may be more susceptible to economic conditions and risks affecting that industry.

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Cyber Security Risk — Failures or breaches of the electronic systems of the Fund, the Fund’s adviser, distributor, and other service providers, or the issuers of securities in which the Fund invests have the ability to cause disruptions and negatively impact the Fund’s business operations, potentially resulting in financial losses to the Fund and its shareholders. While the Fund has established business continuity plans and risk management systems seeking to address system breaches or failures, there are inherent limitations in such plans and systems. Furthermore, the Fund cannot control the cyber security plans and systems of the Fund’s service providers or issuers of securities in which the Fund invests.

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Derivatives Risk — The Fund’s use of derivatives may increase its costs, reduce the Fund’s returns and/or increase volatility. Derivatives involve significant risks, including:

Leverage Risk — The Fund’s use of derivatives can magnify the Fund’s gains and losses. Relatively small market movements may result in large changes in the value of a derivatives position and can result in losses that greatly exceed the amount originally invested.

Market Risk — Some derivatives are more sensitive to interest rate changes and market price fluctuations than other securities. The Fund could also suffer losses related to its derivatives positions as a result of unanticipated market movements, which losses are potentially unlimited. Finally, BFA may not be able to predict correctly the direction of securities prices, interest rates and other economic factors, which could cause the Fund’s derivatives positions to lose value.

Counterparty Risk — Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will be unable or unwilling to fulfill its contractual obligation, and the related risks of having concentrated exposure to such a counterparty.

Illiquidity Risk —The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately.

Operational Risk — The use of derivatives includes the risk of potential operational issues, including documentation issues, settlement issues, systems failures, inadequate controls and human error.

Legal Risk — The risk of insufficient documentation, insufficient capacity or authority of counterparty, or legality or enforceability of a contract.

Volatility and Correlation Risk — The Fund’s use of derivatives may reduce the Fund’s returns and/or increase volatility. Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund’s use of derivatives is that the fluctuations in their values may not correlate with the overall securities markets.

Valuation Risk — Valuation for derivatives may not be readily available in the market. Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Derivatives may also expose the Fund to greater risk and increase its costs. Certain transactions in derivatives involve substantial leverage risk and may expose the Fund to potential losses that exceed the amount originally invested by the Fund.

Hedging Risk — When a derivative is used as a hedge against a position that the Fund holds, any loss generated by the derivative generally should be substantially offset by gains on the hedged investment, and vice versa. While hedging can reduce or eliminate losses, it can also reduce or eliminate gains. Hedges are sometimes subject to imperfect matching between the derivative and the underlying security, and there can be no assurance that the Fund’s hedging transactions will be effective. The use of hedging may result in certain adverse tax consequences noted below.

Tax Risk — The federal income tax treatment of a derivative may not be as favorable as a direct investment in an underlying asset and may adversely affect the timing, character and amount of income the Fund realizes from its investments. As a result, a larger portion of the Fund’s distributions may be treated as ordinary income rather than capital gains. In addition, certain derivatives are subject to mark-to-market or straddle provisions of the Internal Revenue Code of 1986, as amended. If such provisions are applicable, there could be an increase (or decrease) in the amount of taxable dividends paid by the Fund. In addition, the tax treatment of certain derivatives, such as swaps, is unsettled and may be subject to future legislation, regulation or administrative pronouncements issued by the Internal Revenue Service (the “IRS”).

Regulatory Risk — Derivative contracts are subject to regulation under the Dodd-Frank Wall Street Reform and Consumer Protection Act (the “Dodd-Frank Act”) in the United States and under comparable regimes in Europe, Asia and other non-U.S. jurisdictions. Under the Dodd-Frank Act, with respect to uncleared swaps, swap dealers are required to collect variation margin from the Fund and may be required by applicable regulations to collect initial margin from the Fund. Both initial and variation margin may be comprised of cash and/or securities, subject to applicable regulatory haircuts. Shares of investment companies (other than certain money market funds) may not be posted as collateral under applicable regulations. In addition, regulations adopted by global prudential regulators that are now in effect require certain bank-regulated counterparties and certain of their affiliates to include in certain financial contracts, including many derivatives contracts, terms that delay or restrict the rights of counterparties, such as the Fund, to terminate such contracts, foreclose upon collateral, exercise other default rights or restrict transfers of credit support in the event that the counterparty and/or its affiliates are subject to certain types of resolution or insolvency proceedings. The implementation of these requirements with respect to derivatives, as well as regulations under the Dodd-Frank Act regarding clearing, mandatory trading and margining of other derivatives, may increase the costs and risks to the Fund of trading in these instruments and, as a result, may affect returns to investors in the Fund.

Future regulatory developments may impact the Fund’s ability to invest or remain invested in certain derivatives. Legislation or regulation may also change the way in which the Fund itself is regulated. BFA cannot predict the effects of any new governmental regulation that may be implemented on the ability of the Fund to use swaps or any other financial derivative product, and there can be no assurance that any new governmental regulation will not adversely affect the Fund’s ability to achieve its investment objective.

Risks Specific to Certain Derivatives Used by the Fund

Futures — Futures are standardized, exchange-traded contracts that obligate a purchaser to take delivery, and a seller to make delivery, of a specific amount of an asset at a specified future date at a specified price. The primary risks associated with the use of futures contracts and options are: (a) the imperfect correlation between the change in market value of the instruments held by the Fund and the price of the futures contract or option; (b) the possible lack of a liquid secondary market for a futures contract and the resulting inability to close a futures contract when desired; (c) losses caused by unanticipated market movements, which are potentially unlimited; (d) the investment adviser’s inability to predict correctly the direction of securities prices, interest rates, currency exchange rates and other economic factors; and (e) the possibility that the counterparty will default in the performance of its obligations.

Options — An option is an agreement that, for a premium payment or fee, gives the option holder (the purchaser) the right but not the obligation to buy (a “call option”) or sell (a “put option”) the underlying asset (or settle for cash in an amount based on an underlying asset, rate, or index) at a specified price (the “exercise price”) during a period of time or on a specified date. Investments in options are considered speculative. When the Fund purchases an option, it may lose the total premium paid for it if the price of the underlying security or other assets decreased, remained the same or failed to increase to a level at or beyond the exercise price (in the case of a call option) or increased, remained the same or failed to decrease to a level at or below the exercise price (in the case of a put option). If a put or call option purchased by the Fund were permitted to expire without being sold or exercised, its premium would represent a loss to the Fund. To the extent that the Fund writes or sells an option, if the decline or increase in the underlying asset is significantly below or above the exercise price of the written option, the Fund could experience a substantial loss.

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Expense Risk — Fund expenses are subject to a variety of factors, including fluctuations in the Fund’s net assets. Accordingly, actual expenses may be greater or less than those indicated. For example, to the extent that the Fund’s net assets decrease due to market declines or redemptions, the Fund’s expenses will increase as a percentage of Fund net assets. During periods of high market volatility, these increases in the Fund’s expense ratio could be significant.

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Foreign Securities Risk — Because the Fund may invest in companies located in countries other than the United States, the Fund may be exposed to risks associated with foreign investments.

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The value of holdings traded outside the United States (and any hedging transactions in foreign currencies) will be affected by changes in currency exchange rates.

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The costs of non-U.S. securities transactions tend to be higher than those of U.S. transactions.

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Foreign holdings may be adversely affected by foreign government action.

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International trade barriers or economic sanctions against certain non-U.S. countries may adversely affect these holdings.

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The economies of certain countries may compare unfavorably with the U.S. economy.

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Foreign securities markets may be smaller than the U.S. markets, which may make trading more difficult.

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The Fund’s claims to recover foreign withholding taxes may not be successful, which may adversely affect the Fund’s net asset value.

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Illiquid Investments Risk — The Fund may not acquire any illiquid investment if, immediately after the acquisition, the Fund would have invested more than 15% of its net assets in illiquid investments. An illiquid investment is any investment that the Fund reasonably expects cannot be sold or disposed of in current market conditions in seven calendar days or less without the sale or disposition significantly changing the market value of the investment. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. There can be no assurance that a security or instrument that is deemed to be liquid when purchased will continue to be liquid for as long as it is held by the Fund, and any security or instrument held by the Fund may be deemed an illiquid investment pursuant to the Fund’s liquidity risk management program. The Fund’s illiquid investments may reduce the returns of the Fund because it may be difficult to sell the illiquid investments at an advantageous time or price. In addition, if the Fund is limited in its ability to sell illiquid investments during periods when shareholders are redeeming their shares, the Fund will need to sell liquid securities to meet redemption requests and illiquid securities will become a larger portion of the Fund’s holdings. An investment may be illiquid due to, among other things, the reduced number and capacity of traditional market participants to make a market in fixed-income securities or the lack of an active trading market. To the extent that the Fund’s principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to the risks associated with illiquid investments. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other

cash needs, the Fund may suffer a loss. This may be magnified in a rising interest rate environment or other circumstances where investor redemptions from fixed-income mutual funds may be higher than normal. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.

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Large Shareholder and Large-Scale Redemption Risk — Certain shareholders, including a third-party investor, the Fund’s adviser or an affiliate of the Fund’s adviser, or another entity, may from time to time own or manage a substantial amount of Fund shares or may invest in the Fund and hold its investment for a limited period of time. There can be no assurance that any large shareholder or large group of shareholders would not redeem their investment or that the size of the Fund would be maintained. Redemptions of a large number of Fund shares by these shareholders may adversely affect the Fund’s liquidity and net assets. These redemptions may force the Fund to sell portfolio securities to meet redemption requests when it might not otherwise do so, which may negatively impact the Fund’s NAV and increase the Fund’s brokerage costs and/or accelerate the realization of taxable income and cause the Fund to make taxable distributions to its shareholders earlier than the Fund otherwise would have. In addition, under certain circumstances, non-redeeming shareholders may be treated as receiving a disproportionately large taxable distribution during or with respect to such tax year. The Fund also may be required to sell its more liquid Fund investments to meet a large redemption, in which case the Fund’s remaining assets may be less liquid, more volatile, and more difficult to price. In addition, large redemptions can result in the Fund’s current expenses being allocated over a smaller asset base, which generally results in an increase in the Fund’s expense ratio. Because large redemptions can adversely affect a portfolio manager’s ability to implement a fund’s investment strategy, the Fund also reserves the right to redeem in-kind, subject to certain conditions. In addition, large purchases of Fund shares may adversely affect the Fund’s performance to the extent that the Fund is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would, diluting its investment returns.

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Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. As an open-end investment company registered with the Securities and Exchange Commission, the Fund is subject to the federal securities laws, including the Investment Company Act of 1940, as amended (the “Investment Company Act”), and the rules thereunder. Under Rule 18f-4 under the Investment Company Act, among other things, the Fund must either use derivatives in a limited manner or comply with an outer limit on fund leverage risk based on value-at-risk. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet the applicable requirements of the Investment Company Act and the rules thereunder. Increases and decreases in the value of the Fund’s portfolio will be magnified when the Fund uses leverage.

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Operational Risk — The Fund is exposed to operational risks arising from a number of factors, including, but not limited to, human errors, processing and communication errors, errors of the Fund’s service providers, counterparties or other third parties, failed or inadequate internal or external processes, and technology or systems failures. The use of certain investment strategies that involve manual or additional processing, such as over-the-counter derivatives, increases these risks. While service providers are required to have appropriate operational risk management policies and procedures, their methods of operational risk management may differ from those of the Fund in the setting of priorities, the personnel and resources available or the effectiveness of relevant controls. The Fund and BFA seek to reduce these operational risks through controls, procedures and oversight. However, it is not possible to identify all of the operational risks that may affect the Fund or to develop processes and controls that completely eliminate or mitigate the occurrence or effects of such failures. The Fund, including its performance and continued operation, and its shareholders could be negatively impacted as a result.

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Reliance on Advisor Risk — The Fund is dependent upon services and resources provided by BFA, and therefore BFA’s parent, BlackRock, Inc. BFA is not required to devote its full time to the business of the Fund and there is no guarantee or requirement that any investment professional or other employee of BFA will allocate a substantial portion of his or her time to the Fund. The loss of, or changes in, BFA’s personnel could have a negative effect on the performance or the continued operation of the Fund.

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Securities Lending Risk — The Fund may engage in securities lending. Securities lending involves the risk that the Fund may lose money because the borrower of the loaned securities fails to return the securities in a timely manner or at all. The Fund could also lose money in the event of a decline in the value of collateral provided for loaned securities or a decline in the value of any investments made with cash collateral. These events could also trigger adverse tax consequences for the Fund. BlackRock Institutional Trust Company, N.A. (“BTC”), the Fund’s securities lending agent, will consider the tax impact to shareholders of substitute payments for dividends when managing the Fund’s securities lending program.

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Valuation Risk — The price the Fund could receive upon the sale of any particular portfolio investment may differ from the Fund’s valuation of the investment, particularly for securities that trade in thin or volatile markets or that are valued using a fair valuation methodology or a price provided by an independent pricing service. As a result, the price received upon the sale of an investment may be less than the value ascribed by the Fund, and the Fund could realize a greater than expected loss or lesser than expected gain upon the sale of the investment. Pricing services that value fixed-income securities generally utilize a range of market-based and security-specific inputs and assumptions, as well as considerations about general market conditions, to establish a price. Pricing services generally value fixed-income securities assuming orderly transactions of an institutional round lot size, but may be held or transactions may be conducted in such securities in smaller, odd lot sizes. Odd lots may trade at lower prices than institutional round lots. The Fund’s ability to value its investments may also be impacted by technological issues and/or errors by pricing services or other third-party service providers.

For a description of the Fund’s policies and procedures with respect to disclosure of the Master Portfolio’s portfolio holdings and a further discussion of the Fund’s investments and risks, please refer to the Fund’s SAI.

Share Classes at a Glance1

	Investor A Shares	Institutional Shares
Availability	Generally available through Financial Intermediaries.	Limited to certain investors, including: •Individuals and “Institutional Investors,” which include, but are not limited to, endowments, foundations, family offices, local, city and state governmental institutions, corporations and insurance company separate accounts, who may purchase shares of the Fund through a Financial Intermediary that has entered into an agreement with the Distributor to purchase such shares. •Employer-sponsored retirement plans (not including SEP IRAs, SIMPLE IRAs or SARSEPs), state sponsored 529 college savings plans, collective trust funds, investment companies or other pooled investment vehicles, unaffiliated thrifts and unaffiliated banks and trust companies, each of which may purchase shares of the Fund through a Financial Intermediary that has entered into an agreement with the Distributor to purchase such shares. •Employees, officers and directors/trustees of BlackRock or its affiliates and immediate family members of such persons, if they open an account directly with BlackRock. •Participants in certain programs sponsored by BlackRock or its affiliates or other Financial Intermediaries. •Tax-qualified accounts for insurance agents that are registered representatives of an insurance company’s broker-dealer that has entered into an agreement with the Distributor to offer Institutional Shares, and the family members of such persons. •Clients investing through Financial Intermediaries that have entered into an agreement with the Distributor to offer such shares on a platform that charges a transaction based sales commission outside of the Fund. •Clients investing through a self-directed IRA brokerage account program sponsored by a retirement plan record-keeper, provided that such program offers only mutual fund options and that the program maintains an account with the Fund on an omnibus basis.

	Investor A Shares	Institutional Shares
Minimum Investment	$1,000 for all accounts except: •$50, if establishing an Automatic Investment Plan (“AIP”). •There is no investment minimum for employer- sponsored retirement plans (not including SEP IRAs, SIMPLE IRAs or SARSEPs). •There is no investment minimum for certain fee- based programs.	There is no investment minimum for: •Employer-sponsored retirement plans (not including SEP IRAs, SIMPLE IRAs or SARSEPs), state sponsored 529 college savings plans, collective trust funds, investment companies or other pooled investment vehicles, unaffiliated thrifts and unaffiliated banks and trust companies. •Employees, officers and directors/trustees of BlackRock or its affiliates and immediate family members of such persons, if they open an account directly with BlackRock. •Clients of Financial Intermediaries that: (i) charge such clients a fee for advisory, investment consulting, or similar services or (ii) have entered into an agreement with the Distributor to offer Institutional Shares through a no-load program or investment platform. •Clients investing through a self-directed IRA brokerage account program sponsored by a retirement plan record-keeper, provided that such program offers only mutual fund options and that the program maintains an account with the Fund on an omnibus basis. $2 million for individuals and Institutional Investors. $1,000 investment minimum for: •Clients investing through Financial Intermediaries that offer such shares on a platform that charges a transaction based sales commission outside of the Fund. •Tax-qualified accounts for insurance agents that are registered representatives of an insurance company’s broker-dealer that has entered into an agreement with the Distributor to offer Institutional Shares, and the family members of such persons.
Initial Sales Charge?	No. Entire purchase price is invested in shares of the Fund.	No. Entire purchase price is invested in shares of the Fund.
Deferred Sales Charge?	No.	No.
Distribution and Service (12b-1) Fees?	No Distribution Fee. 0.25% Annual Service Fee.	No.
Redemption Fees?	No.	No.
Conversion to Investor A Shares?	N/A	No.
Advantage	Generally available to most investors.	No distribution or shareholder servicing fees.
Disadvantage	You pay an ongoing annual service fee, which means you can expect lower total performance than other share classes with a lower or no service fee.	Limited availability.

Please see “Details About the Share Classes” for more information about each share class.

The following pages will cover the additional details of each share class, including the eligibility requirements to purchase Institutional Shares.

More information about existing sales charge reductions and waivers is available free of charge in a clear and prominent format via hyperlink at www.blackrock.com and in the SAI, which is available on the website or on request.

Details About the Share Classes

Investor A Shares

Investor A Shares of the Fund pay an ongoing service fee of 0.25% per year. The Distributor normally pays the annual 0.25% Investor A Shares service fee to dealers on a monthly basis.

Institutional Shares

Institutional Shares are not subject to any sales charge. Only certain investors are eligible to buy Institutional Shares. Your Financial Intermediary can help you determine whether you are eligible to buy Institutional Shares. The Fund may permit a lower initial investment for certain investors if their purchase, combined with purchases by other investors received together by the Fund, meets the minimum investment requirement.

Institutional Shares may also be available on certain brokerage platforms. An investor transacting in Institutional Shares on such brokerage platforms through a broker acting as an agent for the investor may be required to pay a commission and/or other forms of compensation to the broker. Shares of the Fund are available in other share classes that have different fees and expenses.

Eligible Institutional Shares investors include the following:

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Individuals and “Institutional Investors” with a minimum initial investment of $2 million who may purchase shares of the Fund through a Financial Intermediary that has entered into an agreement with the Distributor to purchase such shares;

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Clients of Financial Intermediaries that: (i) charge such clients a fee for advisory, investment consulting, or similar services or (ii) have entered into an agreement with the Distributor to offer Institutional Shares through a no-load program or investment platform, in each case, with no minimum initial investment;

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Clients investing through Financial Intermediaries that have entered into an agreement with the Distributor to offer such shares on a platform that charges a transaction based sales commission outside of the Fund, with a minimum initial investment of $1,000;

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Employer-sponsored retirement plans (not including SEP IRAs, SIMPLE IRAs or SARSEPs), state sponsored 529 college savings plans, collective trust funds, investment companies or other pooled investment vehicles, unaffiliated thrifts and unaffiliated banks and trust companies, each of which is not subject to any minimum initial investment and may purchase shares of the Fund through a Financial Intermediary that has entered into an agreement with the Distributor to purchase such shares;

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Trust department clients of Bank of America, N.A. and its affiliates for whom they (i) act in a fiduciary capacity (excluding participant directed employee benefit plans); (ii) otherwise have investment discretion; or (iii) act as custodian for at least $2 million in assets, who are not subject to any minimum initial investment;

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Holders of certain Bank of America Corporation (“BofA Corp.”) sponsored unit investment trusts (“UITs”) who reinvest dividends received from such UITs in shares of the Fund, who are not subject to any minimum initial investment;

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Employees, officers and directors/trustees of BlackRock, Inc., mutual funds sponsored and advised by BFA or its affiliates (“BlackRock Funds”), BofA Corp., Barclays PLC or their respective affiliates and immediate family members of such persons, if they open an account directly with BlackRock, who are not subject to any minimum initial investment;

◼

Tax-qualified accounts for insurance agents that are registered representatives of an insurance company’s broker-dealer that has entered into an agreement with the Distributor to offer Institutional Shares, and the family members of such persons; and

◼

Clients investing through a self-directed IRA brokerage account program sponsored by a retirement plan record-keeper, provided that such program offers only mutual fund options and that the program maintains an account with the Fund on an omnibus basis.

The Fund reserves the right to modify or waive the above-stated policies at any time.

Right of Accumulation

Investors have a “right of accumulation” under which any of the following may be combined with the amount of the current purchase in determining whether an investor qualifies for a breakpoint and a reduced front-end sales charge:

The current value of an investor’s existing Investor A and A1, Investor C, Investor P, Institutional, Class K and Premier Shares in most BlackRock Funds,

ii.

The current value of an investor’s existing shares of certain unlisted closed-end management investment companies sponsored and advised by BlackRock or its affiliates and

iii.

The investment in the BlackRock CollegeAdvantage 529 Program by the investor or by or on behalf of the investor’s spouse and children.

Financial Intermediaries may value current holdings of their customers differently for purposes of determining whether an investor qualifies for a breakpoint and a reduced front-end sales charge, although customers of the same Financial Intermediary will be treated similarly. In order to use this right, the investor must alert BlackRock to the existence of any previously purchased shares.

Distribution and Shareholder Servicing Payments

Plan Payments

The Trust, on behalf of the Fund, has adopted a plan (the “Plan”) pursuant to Rule 12b-1 under the Investment Company Act with respect to the Investor Shares that allows the Fund to pay distribution fees for the sale of its shares and/or shareholder servicing fees for certain services provided to its shareholders. Institutional and Investor A Shares do not pay distribution fees.

Under the Plan, the Fund pays shareholder servicing fees (also referred to as general shareholder liaison services fees) to Financial Intermediaries for providing support services to their customers who own Investor Shares of the Fund. The shareholder servicing fee payment is calculated as a percentage of the average daily net asset value of Investor Shares of the Fund. All Investor Shares pay this shareholder servicing fee. Institutional Shares do not pay a shareholder servicing fee.

In return for the shareholder servicing fee, Financial Intermediaries (including BlackRock) may provide one or more of the following services to their customers who own Investor Shares:

◼

Answering customer inquiries regarding account status and history, the manner in which purchases, exchanges and redemptions or repurchases of shares may be effected and certain other matters pertaining to the customers’ investments;

◼

Assisting customers in designating and changing dividend options, account designations and addresses; and

◼

Providing other similar shareholder liaison services.

The shareholder servicing fees payable pursuant to the Plan are paid to compensate Financial Intermediaries for the administration and servicing of shareholder accounts and are not costs which are primarily intended to result in the sale of the Fund’s shares.

Because the fees paid by the Fund under the Plan are paid out of Fund assets on an ongoing basis, over time these fees will increase the cost of your investment and may cost you more than paying other types of sales charges. For more information on the Plan, including a complete list of services provided thereunder, see the SAI.

Other Payments by the Fund

In addition to fees that the Fund may pay to a Financial Intermediary pursuant to the Plan and fees the Fund pays to its transfer agent, BNY Mellon Investment Servicing (US) Inc. (the “Transfer Agent”), BFA, on behalf of the Fund, may enter into non-Plan agreements with affiliated and unaffiliated Financial Intermediaries pursuant to which the Fund will pay a Financial Intermediary for administrative, networking, recordkeeping, sub-transfer agency, sub-accounting and/or shareholder services. These non-Plan payments are generally based on either (1) a percentage of the average daily net assets of Fund shareholders serviced by a Financial Intermediary or (2) a fixed dollar amount for each account serviced by a Financial Intermediary. The aggregate amount of these payments may be substantial.

Other Payments by BFA

From time to time, BFA, the Distributor or their affiliates also may pay a portion of the fees for administrative, networking, recordkeeping, sub-transfer agency, sub-accounting and shareholder services described above at its or their own expense and out of its or their profits. BFA, the Distributor and their affiliates may also compensate affiliated and unaffiliated Financial Intermediaries for the sale and distribution of shares of the Fund. These payments would be in addition to the Fund payments described in this prospectus and may be a fixed dollar amount, may be based on the number of customer accounts maintained by the Financial Intermediary, may be based on a percentage of the value of shares sold to, or held by, customers of the Financial Intermediary or may be calculated on another basis. The aggregate amount of these payments by BFA, the Distributor and their affiliates may be substantial and, in some circumstances, may create an incentive for a Financial Intermediary, its employees or associated persons to recommend or sell shares of the Fund to you.

Please contact your Financial Intermediary for details about payments it may receive from the Fund or from BFA, the Distributor or their affiliates. For more information, see the SAI.

How to Buy, Sell, Exchange and Transfer Shares

The chart on the following pages summarizes how to buy, sell, exchange and transfer shares through your Financial Intermediary. You may also buy, sell, exchange and transfer shares through BlackRock if your account is held directly with BlackRock. To learn more about buying, selling, exchanging or transferring shares through BlackRock, call (800) 441-7762. Because the selection of a mutual fund involves many considerations, your Financial Intermediary may help you with this decision.

With certain limited exceptions, the Fund is generally available only to investors residing in the United States and may not be distributed by a foreign Financial Intermediary. Under this policy, in order to accept new accounts or additional investments (including by way of exchange from another BlackRock Fund) into existing accounts, the Fund generally requires that (i) a shareholder that is a natural person be a U.S. citizen or resident alien, in each case residing within the United States or a U.S. territory (including APO/FPO/DPO addresses), and have a valid U.S. taxpayer identification number, and (ii) a Financial Intermediary or a shareholder that is an entity be domiciled in the United States and have a valid U.S. taxpayer identification number or be domiciled in a U.S. territory and have a valid U.S. taxpayer identification number or IRS Form W-8. Any existing account that is updated to reflect a non-U.S. address will also be restricted from making additional investments.

The Fund may reject any purchase order, modify or waive the minimum initial or subsequent investment requirements for any shareholders and suspend and resume the sale of any share class of the Fund at any time for any reason. In addition, the Fund may waive certain requirements regarding the purchase, sale, exchange or transfer of shares described below.

Under certain circumstances, if no activity occurs in an account within a time period specified by state law, a shareholder’s shares in the Fund may be transferred to that state.

How to Buy Shares

	Your Choices	Important Information for You to Know
Initial Purchase	First, select the share class appropriate for you	Refer to the “Share Classes at a Glance” table in this prospectus (be sure to read this prospectus carefully). When you place your initial order, you must indicate which share class you select (if you do not specify a share class and do not qualify to purchase Institutional Shares, you will receive Investor A Shares). Certain factors, such as the amount of your investment, your time frame for investing and your financial goals, may affect which share class you choose. Your Financial Intermediary can help you determine which share class is appropriate for you.
	Next, determine the amount of your investment	Refer to the minimum initial investment in the “Share Classes at a Glance” table of this prospectus. See “Account Information — Details About the Share Classes” for information on a lower initial investment requirement for certain Fund investors if their purchase, combined with purchases by other investors received together by the Fund, meets the minimum investment requirement.
	Have your Financial Intermediary submit your purchase order	The price of your shares is based on the next calculation of the Fund’s net asset value after your order is placed. Any purchase orders placed prior to the close of business on the New York Stock Exchange (the “NYSE”) (generally 4:00 p.m. Eastern time) will be priced at the net asset value determined that day. Certain Financial Intermediaries, however, may require submission of orders prior to that time. Purchase orders placed after that time will be priced at the net asset value determined on the next business day. A broker-dealer or financial institution maintaining the account in which you hold shares may charge a separate account, service or transaction fee on the purchase or sale of Fund shares that would be in addition to the fees and expenses shown in the Fund’s “Fees and Expenses” table.

	Your Choices	Important Information for You to Know
Initial Purchase (continued)	Have your Financial Intermediary submit your purchase order (continued)	The Fund may reject any order to buy shares and may suspend the sale of shares at any time. Certain Financial Intermediaries may charge a processing fee to confirm a purchase.
	Or contact BlackRock (for accounts held directly with BlackRock)	To purchase shares directly from BlackRock, call (800) 441-7762 and request a new account application. Mail the completed application along with a check payable to “BlackRock Funds” to the Transfer Agent at the address on the application. The Fund limits purchases by personal check to $500,000 per trade.
Add to Your Investment	Purchase additional shares	For Investor A Shares, the minimum investment for additional purchases is generally $50 for all accounts (with the exception of certain employer-sponsored retirement plans which may have a lower minimum for additional purchases). The minimums for additional purchases may be waived under certain circumstances. Institutional Shares have no minimum for additional purchases.
	Have your Financial Intermediary submit your purchase order for additional shares	To purchase additional shares, you may contact your Financial Intermediary.
	Or contact BlackRock (for accounts held directly with BlackRock)	Purchase by Telephone: Call (800) 441-7762 and speak with one of our representatives. The Fund has the right to reject any telephone request for any reason. Purchase in Writing: You may send a written request to BlackRock at the address on the back cover of this prospectus. Purchase by VRU: Investor A Shares may also be purchased by use of the Fund’s automated voice response unit (“VRU”) service at (800) 441-7762. Purchase by Internet: You may purchase your shares and view activity in your account by logging onto the BlackRock website at www.blackrock.com. Purchases made on the Internet using the Automated Clearing House (“ACH”) will have a trade date that is the day after the purchase is made. Certain institutional clients’ purchase orders of Institutional Shares placed by wire prior to the close of business on the NYSE will be priced at the net asset value determined that day. Contact your Financial Intermediary or BlackRock for further information. The Fund limits Internet purchases in shares of the Fund to $25,000 per trade. Different maximums may apply to certain institutional investors. Please read the On-Line Services Disclosure Statement and User Agreement, the Terms and Conditions page and the Consent to Electronic Delivery Agreement (if you consent to electronic delivery), before attempting to transact online. The Fund employs reasonable procedures to confirm that transactions entered over the Internet are genuine. By entering into the User Agreement with the Fund in order to open an account through the website, the shareholder waives any right to reclaim any losses from the Fund or any of its affiliates incurred through fraudulent activity.
	Acquire additional shares by reinvesting dividends and capital gains	All dividends and capital gains distributions are automatically reinvested in shares of the Fund at net asset value. To make any changes to your dividend and/or capital gains distributions options, please call BlackRock at (800) 441-7762 or contact your Financial Intermediary (if your account is not held directly with BlackRock).
	Participate in the AIP	BlackRock’s AIP allows you to invest a specific amount on a periodic basis from your checking or savings account into your investment account. Refer to the “Account Services and Privileges” section of this prospectus for additional information.
How to Pay for Shares	Making payment for purchases	Payment for an order must be made in Federal funds or other immediately available funds by the time specified by your Financial Intermediary, but in no event later than 4:00 p.m. (Eastern time) on the first business day following BlackRock’s receipt of the order. If payment is not received by this time, the order will be canceled and you and your Financial Intermediary will be responsible for any loss to the Fund.

	Your Choices	Important Information for You to Know
How to Pay for Shares (continued)	Making payment for purchases (continued)	For shares purchased directly from the Fund, a check payable to BlackRock Funds which bears the name of the Fund must accompany a completed purchase application. The Fund limits purchases by personal check to $500,000 per trade. There is a $20 fee for each purchase check that is returned due to insufficient funds. The Fund does not accept third-party checks. You may also wire Federal funds to the Fund to purchase shares, but you must call (800) 441-7762 before doing so to confirm the wiring instructions.

How to Sell Shares

	Your Choices	Important Information for You to Know
Full or Partial Redemption of Shares	Have your Financial Intermediary submit your sales order	You can make redemption requests through your Financial Intermediary. Shareholders should indicate whether they are redeeming Investor A or Institutional Shares. The price of your shares is based on the next calculation of the Fund’s net asset value after your order is placed. For your redemption request to be priced at the net asset value on the day of your request, you must submit your request to your Financial Intermediary prior to that day’s close of business on the NYSE (generally, 4:00 p.m. Eastern time). Certain Financial Intermediaries, however, may require submission of orders prior to that time. Any redemption request placed after that time will be priced at the net asset value at the close of business on the next business day. Regardless of the method the Fund uses to make payment of your redemption proceeds (check, wire or ACH), your redemption proceeds typically will be sent one to two business days after your request is submitted, but in any event, within seven days. Certain Financial Intermediaries may charge a fee to process a redemption of shares. The Fund may reject an order to sell shares under certain circumstances.
	Selling shares held directly with BlackRock	Methods of Redeeming Redeem by Telephone: You may sell Investor A Shares held directly with BlackRock by telephone request if certain conditions are met and if the amount being sold is less than (i) $100,000 for payments by check or (ii) $250,000 for payments through ACH or wire transfer. Certain redemption requests, such as those in excess of these amounts, must be in writing with a medallion signature guarantee. Call (800) 441-7762 for details. You can obtain a medallion signature guarantee stamp from a bank, securities dealer, securities broker, credit union, savings and loan association, national securities exchange or registered securities association. A notary public seal will not be acceptable. The Fund, its administrator and the Distributor will employ reasonable procedures to confirm that instructions communicated by telephone are genuine. The Fund and its service providers will not be liable for any loss, liability, cost or expense for acting upon telephone instructions that are reasonably believed to be genuine in accordance with such procedures. The Fund may refuse a telephone redemption request if it believes it is advisable to do so. During periods of substantial economic or market change, telephone redemptions may be difficult to complete. Please find alternative redemption methods below. Redeem by VRU: Investor A Shares may also be redeemed by use of the Fund’s automated VRU service. Payment for Investor A Shares redeemed by the VRU service may be made for non-retirement accounts in amounts up to $25,000, either through check, ACH or wire. Redeem by Internet: You may redeem in your account by logging onto the BlackRock website at www.blackrock.com. Proceeds from Internet redemptions may be sent via check, ACH or wire to the bank account of record. Payment for Investor A Shares redeemed by Internet may be made for non-retirement accounts in amounts up to $25,000, either

	Your Choices	Important Information for You to Know
Full or Partial Redemption of Shares (continued)	Selling shares held directly with BlackRock (continued)	through check, ACH or wire. Different maximums may apply to investors in Institutional Shares. Redeem in Writing: You may sell shares held with BlackRock by writing to BlackRock, P.O. Box 534429, Pittsburgh, Pennsylvania 15253-4429 or for overnight delivery, Attention: 534429, 500 Ross Street 154- 0520, Pittsburgh, Pennsylvania 15262. All shareholders on the account must sign the letter. A medallion signature guarantee will generally be required but may be waived in certain limited circumstances. You can obtain a medallion signature guarantee stamp from a bank, securities dealer, securities broker, credit union, savings and loan association, national securities exchange or registered securities association. A notary public seal will not be acceptable. If you hold stock certificates, return the certificates with the letter. Proceeds from redemptions may be sent via check, ACH or wire to the bank account of record. Payment of Redemption Proceeds Redemption proceeds may be paid by check or, if the Fund has verified banking information on file, through ACH or by wire transfer. Payment by Check: BlackRock will normally mail redemption proceeds within three business days following receipt of a properly completed request, but in any event within seven days. Shares can be redeemed by telephone and the proceeds sent by check to the shareholder at the address on record. Shareholders will pay $15 for redemption proceeds sent by check via overnight mail. You are responsible for any additional charges imposed by your bank for this service. The Fund reserves the right to reinvest any dividend or distribution amounts (e.g., income dividends or capital gains) which you have elected to receive by check should your check be returned as undeliverable or remain uncashed for more than 6 months. No interest will accrue on amounts represented by uncashed checks. Your check will be reinvested in your account at the net asset value next calculated, on the day of the investment. When reinvested, those amounts are subject to the risk of loss like any fund investment. If you elect to receive distributions in cash and a check remains undeliverable or uncashed for more than 6 months, your cash election may also be changed automatically to reinvest and your future dividend and capital gains distributions will be reinvested in the Fund at the net asset value as of the date of payment of the distribution. Payment by Wire Transfer: Payment for redeemed shares for which a redemption order is received before 4:00 p.m. (Eastern time) on a business day is normally made in Federal funds wired to the redeeming shareholder on the next business day, provided that the Fund’s custodian is also open for business. Payment for redemption orders received after 4:00 p.m. (Eastern time) or on a day when the Fund’s custodian is closed is normally wired in Federal funds on the next business day following redemption on which the Fund’s custodian is open for business. The Fund reserves the right to wire redemption proceeds within seven days after receiving a redemption order if, in the judgment of the Fund, an earlier payment could adversely affect the Fund. If a shareholder has given authorization for expedited redemption, shares can be redeemed by Federal wire transfer to a single previously designated bank account. Shareholders will pay $7.50 for redemption proceeds sent by Federal wire transfer. You are responsible for any additional charges imposed by your bank for this service. No charge for wiring redemption payments with respect to Institutional Shares is imposed by the Fund. The Fund is not responsible for the efficiency of the Federal wire system or the shareholder’s firm or bank. To change the name of the single, designated bank account to receive wire redemption proceeds, it is necessary to send a written request to the Fund at the address on the back cover of this prospectus.

	Your Choices	Important Information for You to Know
Full or Partial Redemption of Shares (continued)	Selling shares held directly with BlackRock (continued)	Payment by ACH: Redemption proceeds may be sent to the shareholder’s bank account (checking or savings) via ACH. Payment for redeemed shares for which a redemption order is received before 4:00 p.m. (Eastern time) on a business day is normally sent to the redeeming shareholder the next business day, with receipt at the receiving bank within the next two business days (48-72 hours), provided that the Fund’s custodian is also open for business. Payment for redemption orders received after 4:00 p.m. (Eastern time) or on a day when the Fund’s custodian is closed is normally sent on the next business day following redemption on which the Fund’s custodian is open for business. The Fund reserves the right to send redemption proceeds within seven days after receiving a redemption order if, in the judgment of the Fund, an earlier payment could adversely affect the Fund. No charge for sending redemption payments via ACH is imposed by the Fund. *** If you make a redemption request before the Fund has collected payment for the purchase of shares, the Fund may delay mailing your proceeds. This delay will usually not exceed ten days.
Redemption Proceeds		Under normal circumstances, the Fund expects to meet redemption requests by using cash or cash equivalents in its portfolio or by selling portfolio assets to generate cash. During periods of stressed market conditions, when a significant portion of the Fund’s portfolio may be comprised of less-liquid investments, the Fund may be more likely to limit cash redemptions and may determine to pay redemption proceeds by (i) borrowing under a line of credit it has entered into with a group of lenders, (ii) borrowing from another BlackRock Fund pursuant to an interfund lending program, to the extent permitted by the Fund’s investment policies and restrictions as set forth in the SAI, and/or (iii) transferring portfolio securities in-kind to you. The SAI includes more information about the Fund’s line of credit and interfund lending program, to the extent applicable. If the Fund pays redemption proceeds by transferring portfolio securities in-kind to you, you may pay transaction costs to dispose of the securities, and you may receive less for them than the price at which they were valued for purposes of redemption.

How to Exchange Shares or Transfer Your Account

	Your Choices	Important Information for You to Know
Exchange Privilege	Selling shares of one BlackRock Fund to purchase shares of another BlackRock Fund (“exchanging”)	Investor or Institutional Shares of the Fund are generally exchangeable for shares of the same class of another BlackRock Fund, to the extent such shares are offered by your Financial Intermediary. You can exchange $1,000 or more of Investor Shares from one fund into the same class of another fund which offers that class of shares (you can exchange less than $1,000 of Investor Shares if you already have an account in the fund into which you are exchanging). Investors who currently own Institutional Shares of the Fund may make exchanges into Institutional Shares of other BlackRock Funds except for investors holding shares through certain client accounts at Financial Intermediaries that are omnibus with the Fund and do not meet applicable minimums. There is no required minimum amount with respect to exchanges of Institutional Shares. You may only exchange into a share class and fund that are open to new investors or in which you have a current account if the fund is closed to new investors. Some of the BlackRock Funds impose a different initial or deferred sales charge schedule. Therefore the exchange of Investor A Shares may be subject to that sales charge. Investor A Shares of the Fund that were obtained with the exchange privilege and that originally were shares of a BlackRock Fund that were subject to a sales charge can be exchanged for Investor A Shares of another BlackRock Fund based on their respective NAVs. The contingent deferred sales charge will

	Your Choices	Important Information for You to Know
Exchange Privilege (continued)	Selling shares of one BlackRock Fund to purchase shares of another BlackRock Fund (“exchanging”) (continued)	continue to be measured from the date of the original purchase. The contingent deferred sales charge schedule applicable to your original purchase will apply to the shares you receive in the exchange and any subsequent exchange. To exercise the exchange privilege, you may contact your Financial Intermediary. Alternatively, if your account is held directly with BlackRock, you may: (i) call (800) 441-7762 and speak with one of our representatives, (ii) make the exchange via the Internet by accessing your account online at www.blackrock.com, or (iii) send a written request to the Fund at the address on the back cover of this prospectus. Please note, if you indicated on your new account application that you did not want the Telephone Exchange Privilege, you will not be able to place exchanges via the telephone until you update this option either in writing or by calling (800) 441-7762. The Fund has the right to reject any telephone request for any reason. Although there is currently no limit on the number of exchanges that you can make, the exchange privilege may be modified or terminated at any time in the future. The Fund may suspend or terminate your exchange privilege at any time for any reason, including if the Fund believes, in its sole discretion, that you are engaging in market timing activities. See “Short-Term Trading Policy” below. For U.S. federal income tax purposes, a share exchange is a taxable event and a capital gain or loss may be realized. Please consult your tax adviser or other Financial Intermediary before making an exchange request.
Transfer Shares to Another Financial Intermediary	Transfer to a participating Financial Intermediary	You may transfer your shares of the Fund only to another Financial Intermediary that has entered into an agreement with the Distributor. Certain shareholder services may not be available for the transferred shares. All future trading of these assets must be coordinated by the receiving firm. If your account is held directly with BlackRock, you may call (800) 441-7762 with any questions; otherwise please contact your Financial Intermediary to accomplish the transfer of shares.
	Transfer to a non-participating Financial Intermediary	You must either: •Transfer your shares to an account with the Fund; or •Sell your shares. If your account is held directly with BlackRock, you may call (800) 441-7762 with any questions; otherwise please contact your Financial Intermediary to accomplish the transfer of shares.

Account Services and Privileges

The following table provides examples of account services and privileges available in your BlackRock account. Certain of these account services and privileges are only available to shareholders of Investor A Shares whose accounts are held directly with BlackRock. If your account is held directly with BlackRock, please call (800) 441-7762 or visit www.blackrock.com for additional information as well as forms and applications. Otherwise, please contact your Financial Intermediary for assistance in requesting one or more of the following services and privileges.

Automatic Investment Plan	Allows systematic investments on a periodic basis from your checking or savings account.	BlackRock’s AIP allows you to invest a specific amount on a periodic basis from your checking or savings account into your investment account. You may apply for this option upon account opening or by completing the AIP application. The minimum investment amount for an automatic investment is $50 per portfolio.
Dividend Allocation Plan	Automatically invests your distributions into another BlackRock Fund of your choice pursuant to your instructions, without any fees or sales charges.	Dividend and capital gains distributions may be reinvested in your account to purchase additional shares or paid in cash. Using the Dividend Allocation Plan, you can direct your distributions to your bank account (checking or savings), to purchase shares of another fund at BlackRock without any fees or sales charges, or by check to a special payee. Please call (800) 441-7762 for details. If investing in another BlackRock Fund, the receiving fund must be open to new purchases.

EZ Trader	Allows an investor to purchase or sell Investor A Shares by telephone or over the Internet through ACH.	(NOTE: This option is offered to shareholders whose accounts are held directly with BlackRock. Please speak with your Financial Intermediary if your account is held elsewhere.) Prior to establishing an EZ Trader account, please contact your bank to confirm that it is a member of the ACH system. Once confirmed, complete an application, making sure to include the appropriate bank information, and return the application to the address listed on the form. Prior to placing a telephone or Internet purchase or sale order, please call (800) 441-7762 to confirm that your bank information has been updated on your account. Once this is established, you may place your request to sell shares with the Fund by telephone or Internet. Proceeds will be sent to your pre-designated bank account.
Systematic Exchange Plan	This feature can be used by investors to systematically exchange money from one fund to up to four other funds.	A minimum of $10,000 in the initial BlackRock Fund is required, and investments in any additional funds must meet minimum initial investment requirements.
Systematic Withdrawal Plan (“SWP”)	This feature can be used by investors who want to receive regular distributions from their accounts.	To start a SWP, a shareholder must have a current investment of $10,000 or more in a BlackRock Fund. Shareholders can elect to receive cash payments of $50 or more at any interval they choose. Shareholders may sign up by completing the SWP Application Form, which may be obtained from BlackRock. Shareholders should realize that if withdrawals exceed income the invested principal in their account will be depleted. To participate in the SWP, shareholders must have their dividends reinvested. Shareholders may change or cancel the SWP at any time, with a minimum of 24 hours’ notice. If a shareholder purchases additional Investor A Shares of a fund at the same time he or she redeems shares through the SWP, that investor may lose money because of any applicable sales charge. Ask your Financial Intermediary for details.

Fund’s Rights

The Fund may:

◼

Suspend the right of redemption if trading is halted or restricted on the NYSE or under other emergency conditions described in the Investment Company Act;

◼

Postpone the date of payment upon redemption if trading is halted or restricted on the NYSE or under other emergency conditions described in the Investment Company Act or if a redemption request is made before the Fund has collected payment for the purchase of shares;

◼

Redeem shares for property other than cash as may be permitted under the Investment Company Act; and

◼

Redeem shares involuntarily in certain cases, such as when the value of a shareholder account falls below a specified level.

Note on Low Balance Accounts. Because of the high cost of maintaining smaller shareholder accounts, BlackRock has set a minimum balance of $500 in each Fund position you hold within your account (the “Fund Minimum”), and may redeem the shares in your account if the net asset value of those shares in your account falls below $500 for any reason, including market fluctuation.

You will be notified that the value of your account is less than the Fund Minimum before the Fund makes any involuntary redemption. This notification will provide you with a 90 calendar day period to make an additional investment in order to bring the value of your account to at least $500 before the Fund makes an involuntary redemption. This involuntary redemption will not charge any deferred sales charge, and may not apply to accounts of certain employer-sponsored retirement plans (not including IRAs), qualified state tuition plan (529 Plan) accounts, and select fee-based programs at your Financial Intermediary.

Participation in Fee-Based Programs

If you participate in certain fee-based programs offered by BFA or an affiliate of BFA, or by Financial Intermediaries that have agreements with the Distributor or in certain fee-based programs in which BFA participates, you may be able to buy Institutional Shares, including by exchanges from other share classes. Sales charges on the shares being exchanged may be reduced or waived under certain circumstances. You generally cannot transfer shares held through a fee-based program into another account. Instead, if you choose to leave the fee-based program, you may have to redeem your shares held through the program and purchase shares of another class, which may be subject to distribution and service fees. This may be a taxable event and you may pay any applicable sales charges or redemption fee. Please speak to your Financial Intermediary for information about specific policies and procedures applicable to your account.

Generally, upon termination of a fee-based program, the shares may be liquidated, or the shares can be held in an account. In certain instances, when a shareholder chooses to continue to hold the shares, whatever share class was held in the program can be held after termination. Shares that have been held for less than specified periods within the program may be subject to a fee upon redemption. Shareholders that held Investor A or Institutional Shares in the program may be eligible to purchase additional shares of the respective share class of the Fund, but may be subject to upfront sales charges with respect to Investor A Shares. Additional purchases of Institutional Shares are permitted only if you have an existing position at the time of purchase or are otherwise eligible to purchase Institutional Shares. Please speak to your Financial Intermediary for more information.

Certain Financial Intermediaries may, in connection with a change in account type (for example, due to leaving a fee-based program or upon termination of the fee-based program) or otherwise in accordance with the Financial Intermediary’s policies and procedures, exchange the share class held in the program for another share class of the same fund, provided that the exchanged shares are not subject to a sales charge and the shareholder meets the eligibility requirements of the new share class. Please speak to your Financial Intermediary for information about specific policies and procedures applicable to your account.

Details about the features of each fee-based program and the relevant charges, terms and conditions are included in the client agreement for each fee-based program and are available from your Financial Intermediary. Please speak to your Financial Intermediary for more information.

Short-Term Trading Policy

The Boards of Trustees of the Trust and MIP have determined that the interests of long-term shareholders and the Fund’s ability to manage its investments may be adversely affected when shares are repeatedly bought, sold or exchanged in response to short-term market fluctuations — also known as “market timing.” The Fund is not designed for market timing organizations or other entities using programmed or frequent purchases and sales or exchanges. The exchange privilege is not intended as a vehicle for short-term trading. Excessive purchase and sale or exchange activity may interfere with portfolio management, increase expenses and taxes and may have an adverse effect on the performance of the Fund and its returns to shareholders. For example, large flows of cash into and out of the Fund may require the management team to allocate a significant amount of assets to cash or other short-term investments or sell securities, rather than maintaining such assets in securities selected to achieve the Fund’s investment objective. Frequent trading may cause the Fund to sell securities at less favorable prices, and transaction costs, such as brokerage commissions, can reduce the Fund’s performance.

A fund’s investment in non-U.S. securities is subject to the risk that an investor may seek to take advantage of a delay between the change in value of the fund’s portfolio securities and the determination of the fund’s net asset value as a result of different closing times of U.S. and non-U.S. markets by buying or selling fund shares at a price that does not reflect their true value. A similar risk exists for funds that invest in securities of small capitalization companies, securities of issuers located in emerging markets or high yield securities (“junk bonds”) that are thinly traded and therefore may have actual values that differ from their market prices. This short-term arbitrage activity can reduce the return received by long-term shareholders. The Fund will seek to eliminate these opportunities by using fair value pricing, as described in “Management of the Fund – Valuation of Fund Investments” below.

The Fund discourages market timing and seeks to prevent frequent purchases and sales or exchanges of Fund shares that it determines may be detrimental to the Fund or long-term shareholders. The Boards of Trustees of the Trust and MIP have approved the policies discussed below to seek to deter market timing activity. The Boards of Trustees of the Trust and MIP have not adopted any specific numerical restrictions on purchases, sales and exchanges of Fund shares because certain legitimate strategies will not result in harm to the Fund or its shareholders.

If as a result of its own investigation, information provided by a Financial Intermediary or other third party, or otherwise, the Fund believes, in its sole discretion, that your short-term trading is excessive or that you are engaging in market timing activity, it reserves the right to reject any specific purchase or exchange order. If the Fund rejects your purchase or exchange order, you will not be able to execute that transaction, and the Fund will not be responsible for any losses you therefore may suffer. For transactions placed directly with the Fund, the Fund may consider the trading history of accounts under common ownership or control for the purpose of enforcing these policies.

Transactions placed through the same Financial Intermediary on an omnibus basis may be deemed part of a group for the purpose of this policy and may be rejected in whole or in part by the Fund. Certain accounts, such as omnibus accounts and accounts at Financial Intermediaries, however, include multiple investors and such accounts typically provide the Fund with net purchase or redemption and exchange requests on any given day where purchases, redemptions and exchanges of shares are netted against one another and the identity of individual purchasers, redeemers and exchangers whose orders are aggregated may not be known by the Fund. While the Fund monitors for market timing activity, the Fund may be unable to identify such activities because the netting effect in omnibus accounts often makes it more difficult to locate and eliminate market timers from the Fund. The Distributor has entered into agreements with respect to Financial Intermediaries that maintain omnibus accounts with the Fund pursuant to which such Financial Intermediaries undertake to cooperate with the Distributor in monitoring purchase, exchange and redemption orders by their customers in order to detect and prevent short-term or excessive trading in the Fund’s shares through such accounts. Identification of market timers may also be limited by operational systems and technical limitations. In the event that a Financial Intermediary is determined by the Fund to be engaged in market timing or other improper trading activity, the Distributor may terminate such Financial Intermediary’s agreement with the Distributor, suspend such Financial Intermediary’s trading privileges or take other appropriate actions.

There is no assurance that the methods described above will prevent market timing or other trading that may be deemed abusive.

The Fund may from time to time use other methods that it believes are appropriate to deter market timing or other trading activity that may be detrimental to the Fund or long-term shareholders.

Master/Feeder Mutual Fund Structure

The Fund does not have its own investment adviser. Instead, the Fund invests all of its assets in the Master Portfolio, which has an investment objective, strategies and policies substantially identical to those of the Fund. BFA serves as investment adviser to the Master Portfolio. The Master Portfolio may accept investments from other feeder funds. Certain actions involving other feeder funds, such as a substantial withdrawal, could affect the Master Portfolio and, therefore, the Fund.

Feeder Fund Expenses

Feeder funds, including the Fund, bear their respective master portfolio’s expenses in proportion to the amount of assets each invests in the master portfolio. The feeder fund can set its own transaction minimums, fund-specific expenses and conditions.

Feeder Fund Rights

Under the master/feeder structure, the Trust’s Board of Trustees retains the right to withdraw the Fund’s assets from its Master Portfolio if it believes doing so is in the best interests of the Fund’s shareholders. If the Trust’s Board of Trustees decides to withdraw the Fund’s assets, it would then consider whether the Fund should hire its own investment adviser, invest in another master portfolio or take other action.

Management of the Fund

Investment Adviser

The Fund is a “feeder” fund that invests all of its assets in the Master Portfolio, which has the same investment objective, strategies and policies as the Fund. BFA, a registered investment adviser, serves as investment adviser to the Master Portfolio. BFA manages the investment of the Master Portfolio’s assets and provides the Master Portfolio with investment guidance and policy direction in connection with daily portfolio management, subject to the supervision of MIP’s Board of Trustees.

BFA is located at 400 Howard Street, San Francisco, California 94105. BFA is an indirect wholly-owned subsidiary of BlackRock, Inc. As of March 31, 2024, BFA and its affiliates had approximately $10.472 trillion in investment company and other portfolio assets under management.

BFA receives as compensation for its services to the Master Portfolio a management fee equal to 0.04% of the Master Portfolio’s average daily net assets.

BFA has contractually agreed to waive the management fee with respect to any portion of the Master Portfolio’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BFA or its affiliates that have a contractual management fee, through June 30, 2025. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of MIP or by a vote of a majority of the outstanding voting securities of the Master Portfolio.

BFA has contractually agreed to waive its management fees by the amount of investment advisory fees the Master Portfolio pays to BFA indirectly through its investment in affiliated money market funds, through June 30, 2025.

For the fiscal year ended December 31, 2023, BFA received management fees, net of any applicable waivers and/or reimbursements, at the annual rate of 0.04% of the Master Portfolio’s average daily net assets.

A discussion regarding the basis for approval by the Trust’s Board of Trustees and MIP’s Board of Trustees of the investment advisory agreement with BFA is available in the Fund’s semi-annual shareholder report for the period ended June 30, 2023.

From time to time, a manager, analyst, or other employee of BFA or its affiliates may express views regarding a particular asset class, company, security, industry, or market sector. The views expressed by any such person are the views of only that individual as of the time expressed and do not necessarily represent the views of BFA or any other person within the BFA organization. Any such views are subject to change at any time based upon market or other conditions and BFA disclaims any responsibility to update such views. These views may not be relied on as investment advice and, because investment decisions for the Fund are based on numerous factors, may not be relied on as an indication of trading intent on behalf of the Fund.

Portfolio Managers

Information regarding the portfolio managers of the Master Portfolio is set forth below. Further information regarding the portfolio managers, including other accounts managed, compensation, ownership of Fund shares, and possible conflicts of interest, is available in the Fund’s SAI.

Portfolio Manager	Primary Role	Since	Title and Recent Biography
James Mauro	Jointly and primarily responsible for the day-to-day management of the Master Portfolio, including setting the Master Portfolio’s overall investment strategy and overseeing the management of the Master Portfolio.	2011	Head of Index Fixed Income Portfolio Management in the Americas at BlackRock, Inc. since 2020; Managing Director of BlackRock, Inc. since 2015; Director of BlackRock, Inc. from 2010 to 2014; Vice President of State Street Global Advisors from 2001 to 2010.

Portfolio Manager	Primary Role	Since	Title and Recent Biography
Karen Uyehara	Jointly and primarily responsible for the day-to-day management of the Master Portfolio, including setting the Master Portfolio’s overall investment strategy and overseeing the management of the Master Portfolio.	2020	Deputy Head of Index Fixed Income Portfolio Management in the Americas at BlackRock, Inc. since 2020; Managing Director of BlackRock, Inc. since 2020; Director of BlackRock, Inc. from 2010 to 2019; Portfolio Manager of Western Asset Management Company from 2005 to 2010.

Administrative Services

BAL provides the following services, among others, as the Fund’s administrator:

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Supervises the Fund’s administrative operations;

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Provides or causes to be provided management reporting and treasury administration services;

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Financial reporting;

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Legal, blue sky and tax services;

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Preparation of proxy statements and shareholder reports; and

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Engaging and supervising the shareholder servicing agents on behalf of the Fund.

BAL is entitled to receive fees for these services at the annual rate of 0.06% of the average daily net assets of the Fund attributable to Investor A Shares and Institutional Shares. In addition to performing these services, BAL has agreed to bear all costs of operating the Fund, other than brokerage expenses, advisory fees, any 12b-1 fees, certain fees and expenses related to the Trust’s trustees who are not “interested persons” of the Fund or the Trust as defined in the Investment Company Act, and their counsel, auditing fees, litigation expenses, taxes and extraordinary expenses.

Conflicts of Interest

The investment activities of BFA and its affiliates (including BlackRock, Inc. and its subsidiaries (collectively, the “Affiliates”)), and their respective directors, officers or employees, in the management of, or their interest in, their own accounts and other accounts they manage, may present conflicts of interest that could disadvantage the Fund and its shareholders.

BFA and its Affiliates provide investment management services to other funds and discretionary managed accounts that may follow investment programs similar to that of the Fund. BFA and its Affiliates are involved worldwide with a broad spectrum of financial services and asset management activities and may engage in the ordinary course of business in activities in which their interests or the interests of their clients may conflict with those of the Fund. BFA or one or more Affiliates act or may act as an investor, research provider, investment manager, commodity pool operator, commodity trading advisor, financier, underwriter, adviser, trader, lender, index provider, agent and/or principal, and have other direct and indirect interests in securities, currencies, commodities, derivatives and other instruments in which the Fund may directly or indirectly invest. The Fund may invest in securities of, or engage in other transactions with, companies with which an Affiliate has significant debt or equity investments or other interests. The Fund may also invest in issuances (such as structured notes) by entities for which an Affiliate provides and is compensated for cash management services relating to the proceeds from the sale of such issuances. The Fund also may invest in securities of, or engage in other transactions with, companies for which an Affiliate provides or may in the future provide research coverage. An Affiliate may have business relationships with, and purchase, or distribute or sell services or products from or to, distributors, consultants or others who recommend the Fund or who engage in transactions with or for the Fund, and may receive compensation for such services. BFA or one or more Affiliates may engage in proprietary trading and advise accounts and funds that have investment objectives similar to those of the Fund and/or that engage in and compete for transactions in the same types of securities, currencies and other instruments as the Fund. This may include transactions in securities issued by other open-end and closed-end investment companies (which may include investment companies that are affiliated with the Fund and BFA, to the extent permitted under the Investment Company Act). The trading activities of BFA and these Affiliates are carried out without reference to positions held directly or indirectly by the Fund and may result in BFA or an Affiliate having positions in certain securities that are senior or junior to, or have interests different from or adverse to, the securities that are owned by the Fund.

Neither BFA nor any Affiliate is under any obligation to share any investment opportunity, idea or strategy with the Fund. As a result, an Affiliate may compete with the Fund for appropriate investment opportunities. The results of the Fund’s investment activities, therefore, may differ from those of an Affiliate and of other accounts managed by BFA or an Affiliate, and it is possible that the Fund could sustain losses during periods in which one or more Affiliates and other accounts achieve profits on their trading for proprietary or other accounts. The opposite result is also possible.

In addition, the Fund may, from time to time, enter into transactions in which BFA or an Affiliate or their directors, officers or employees or other clients have an adverse interest. Furthermore, transactions undertaken by clients advised or managed by BFA or its Affiliates may adversely impact the Fund. Transactions by one or more clients or BFA or its Affiliates or their directors, officers or employees, may have the effect of diluting or otherwise disadvantaging the values, prices or investment strategies of the Fund. The Fund’s activities may be limited because of regulatory restrictions applicable to BFA or one or more Affiliates and/or their internal policies designed to comply with such restrictions.

Under a securities lending program approved by MIP’s Board of Trustees, MIP, on behalf of the Master Portfolio, has retained BTC, an Affiliate of BFA, to serve as the securities lending agent for the Master Portfolio to the extent that the Master Portfolio participates in the securities lending program. For these services, the securities lending agent will receive a fee from the Master Portfolio based on the returns earned on the Master Portfolio’s lending activities, including investment of the cash received as collateral for the loaned securities. In addition, one or more Affiliates may be among the entities to which the Master Portfolio may lend its portfolio securities under the securities lending program.

The activities of BFA and its Affiliates and their respective directors, officers or employees, may give rise to other conflicts of interest that could disadvantage the Fund and its shareholders. BFA has adopted policies and procedures designed to address these potential conflicts of interest. See the SAI for further information.

Valuation of Fund Investments

When you buy shares, you pay the net asset value. This is the offering price. Shares are also redeemed at their net asset value, minus any applicable redemption fee. The net asset value used in determining your share price is the next one calculated after your purchase or redemption order is received. The net asset value of each class of shares normally is determined once daily Monday through Friday, generally as of the close of regular trading hours of the New York Stock Exchange (“NYSE”) (normally 4:00 p.m., Eastern time), on each day that the NYSE is open for trading, based on prices at the time of closing, provided that any Fund assets or liabilities denominated in currencies other than the U.S. dollar are translated into U.S. dollars at the prevailing market rates on the date of valuation as quoted by one or more data service providers. The net asset value of shares is calculated by dividing the value of the net assets of each class of shares (i.e., the value of its total assets less total liabilities) by the total number of outstanding shares of the class, generally rounded to the nearest cent.

Generally, Institutional Shares will have the highest net asset value because that class has the lowest expenses.

The value of the securities and other assets and liabilities held by the Fund are determined pursuant to BFA’s valuation policies and procedures. BFA has been designated by the Board as the valuation designee for the Fund pursuant to Rule 2a-5 under the Investment Company Act.

Equity securities and other equity instruments for which market quotations are readily available are valued at market value, which is generally determined using the last reported official closing price or, if a reported closing price is not available, the last traded price on the exchange or market on which the security or instrument is primarily traded at the time of valuation. Shares of underlying open-end funds (including money market funds) are valued at net asset value. Shares of underlying exchange-traded closed-end funds or other ETFs are valued at their most recent closing price.

The Fund values fixed-income portfolio securities using last available bid prices or current market quotations provided by dealers or prices (including evaluated prices) supplied by the Fund’s approved independent third-party pricing services, each in accordance with BFA’s valuation policies and procedures. Pricing services may use matrix pricing or valuation models that utilize certain inputs and assumptions to derive values. Pricing services generally value fixed-income securities assuming orderly transactions of an institutional round lot size, but the Fund may hold or transact in such securities in smaller odd lot sizes. Odd lots may trade at lower prices than institutional round lots. An amortized cost method of valuation may be used with respect to debt obligations with 60 days or less remaining to maturity unless BFA determines in good faith that such method does not represent fair value.

Generally, trading in non-U.S. securities, U.S. government securities, money market instruments and certain fixed-income securities is substantially completed each day at various times prior to the close of business on the NYSE. The values of such securities used in computing the net asset value of the Fund’s shares are determined as of such times.

When market quotations are not readily available or are believed by BFA to be unreliable, BFA will fair value the Fund’s investments in accordance with its policies and procedures. BFA may conclude that a market quotation is not readily available or is unreliable if a security or other asset or liability does not have a price source due to its lack of trading or other reasons, if a market quotation differs significantly from recent price quotations or otherwise no longer appears to reflect fair value, where the security or other asset or liability is thinly traded, when there is a significant event subsequent to the most recent market quotation, or if the trading market on which a security is listed is suspended or closed and no appropriate alternative trading market is available. A “significant event” is deemed to occur if BFA determines, in its reasonable business judgment prior to or at the time of pricing the Fund’s assets or liabilities, that the event is likely to cause a material change to the closing market price of one or more assets held by, or liabilities of, the Fund.

For certain foreign assets, a third-party vendor supplies evaluated, systematic fair value pricing based upon the movement of a proprietary multi-factor model after the relevant foreign markets have closed. This systematic fair value pricing methodology is designed to correlate the prices of foreign assets following the close of the local markets to the price that might have prevailed as of the Fund’s pricing time.

Fair value represents a good faith approximation of the value of an asset or liability. The fair value of an asset or liability held by the Fund is the amount the Fund might reasonably expect to receive from the current sale of that asset or the cost to extinguish that liability in an arm’s-length transaction. Valuing the Fund’s investments using fair value pricing will result in prices that may differ from current market valuations and that may not be the prices at which those investments could have been sold during the period in which the particular fair values were used. Use of fair value prices and certain current market valuations could result in a difference between the prices used to calculate the Fund’s net asset value and the prices used by the Underlying Index, which, in turn, could result in a difference between the Fund’s performance and the performance of the Underlying Index.

The Fund may accept orders from certain authorized Financial Intermediaries or their designees. The Fund will be deemed to receive an order when accepted by the Financial Intermediary or designee, and the order will receive the net asset value next computed by the Fund after such acceptance. If the payment for a purchase order is not made by a designated later time, the order will be canceled and the Financial Intermediary could be held liable for any losses.

Dividends, Distributions and Taxes

BUYING A DIVIDEND

Unless your investment is in a tax-deferred account, you may want to avoid buying shares shortly before the Fund pays a dividend. The reason? If you buy shares when the Fund has declared but not yet distributed ordinary income or capital gains, you will pay the full price for the shares and then receive a portion of the price back in the form of a taxable dividend. Before investing you may want to consult your tax adviser.

The Fund will distribute net investment income, if any, at least monthly and net realized capital gains, if any, at least annually. The Fund may also pay a special distribution at the end of the calendar year to comply with federal tax requirements. Dividends may be reinvested automatically in shares of the Fund at net asset value or may be taken in cash. If you would like to receive dividends in cash, contact your Financial Intermediary or the Fund. Although this cannot be predicted with any certainty, the Fund anticipates that a significant amount of its dividends, if any, will consist of ordinary income. Capital gains may be taxable to you at different rates depending on how long the Fund held the assets sold.

You will pay tax on dividends from the Fund whether you receive them in cash or additional shares. If you redeem Fund shares or exchange them for shares of another fund, you generally will be treated as having sold your shares and any gain on the transaction may be subject to tax. Fund distributions derived from qualified dividend income, which consists of dividends received from U.S. corporations and qualifying foreign corporations, and from long-term capital gains are eligible for taxation at a maximum rate of 15% or 20% for individuals, depending on whether their income exceeds certain threshold amounts, which are adjusted annually for inflation.

A 3.8% Medicare tax is imposed on the net investment income (which includes, but is not limited to, interest, dividends and net gain from investments) of U.S. individuals with income exceeding $200,000, or $250,000 if married filing jointly, and of trusts and estates.

Your dividends and redemption proceeds will be subject to backup withholding tax if you have not provided a taxpayer identification number or social security number or the number you have provided is incorrect.

If you are neither a tax resident nor a citizen of the United States or if you are a foreign entity (other than a pass-through entity to the extent owned by U.S. persons), the Fund’s ordinary income dividends will generally be subject to a 30% U.S. withholding tax, unless a lower treaty rate applies. However, certain distributions reported by the Fund as capital gain dividends, interest-related dividends or short-term capital gain dividends and paid to a foreign shareholder may be eligible for an exemption from U.S. withholding tax.

Separately, a 30% withholding tax is currently imposed on U.S.-source dividends, interest and other income items paid to (i) certain foreign financial institutions and investment funds, and (ii) certain other foreign entities. To avoid withholding, foreign financial institutions and investment funds will generally either need to (a) collect and report to the IRS detailed information identifying their U.S. accounts and U.S. account holders, comply with due diligence procedures for identifying U.S. accounts and withhold tax on certain payments made to noncomplying foreign entities and account holders or (b) if an intergovernmental agreement is entered into and implementing legislation is adopted, comply with the agreement and legislation. Other foreign entities will generally either need to provide detailed information identifying each substantial U.S. owner or certify there are no such owners.

This section summarizes some of the consequences under current federal tax law of an investment in the Fund. It is not a substitute for individualized tax advice. Consult your tax adviser about the potential tax consequences of an investment in the Fund under all applicable tax laws.

Financial Highlights

The Financial Highlights tables are intended to help you understand the Fund’s financial performance for the past five years for Institutional Shares and Investor A Shares of the Fund. Certain information reflects the financial results for a single Fund share. The total returns in the tables represent the rate of return that an investor would have earned or lost on an investment in the Fund (assuming reinvestment of all dividends and/or distributions). The information has been audited by PricewaterhouseCoopers LLP, whose report, along with the Fund’s financial statements, is included in the Fund’s Annual Report, which is available upon request.

	iShares U.S. Aggregate Bond Index Fund
	Institutional
	Year Ended 12/31/23	Year Ended 12/31/22	Year Ended 12/31/21	Year Ended 12/31/20	Year Ended 12/31/19
(For a share outstanding throughout each period)	Year Ended 12/31/23	Year Ended 12/31/22	Year Ended 12/31/21	Year Ended 12/31/20	Year Ended 12/31/19
Net asset value, beginning of year	$8.89	$10.48	$10.89	$10.39	$9.85
Net investment income(a)	0.28	0.20	0.19	0.24	0.29
Net realized and unrealized gain (loss)	0.21	(1.56 )	(0.39 )	0.54	0.54
Net increase (decrease) from investment operations	0.49	(1.36 )	(0.20 )	0.78	0.83
Distributions(b)
From net investment income	(0.28 )	(0.23 )	(0.21 )	(0.26 )	(0.29 )
From net realized gain	—	—	(0.00 )(c)	(0.02 )	—
Return of capital	(0.00 )(c)	—	(0.00 )(c)	—	—
Total distributions	(0.28 )	(0.23 )	(0.21 )	(0.28 )	(0.29 )
Net asset value, end of year	$9.10	$8.89	$10.48	$10.89	$10.39
Total Return(d)
Based on net asset value	5.66 %	(13.09 )%	(1.85 )%	7.59 %	8.56 %(e)
Ratios to Average Net Assets(f)(g)
Total expenses	0.10 %	0.10 %	0.09 %	0.09 %	0.10 %
Total expenses after fees waived and/or reimbursed	0.10 %	0.10 %	0.09 %	0.09 %	0.10 %
Net investment income	3.16 %	2.13 %	1.75 %	2.18 %	2.85 %
Supplemental Data
Net assets, end of year (000)	$226,387	$248,151	$419,040	$371,074	$187,854
Portfolio turnover rate of the Master Portfolio(h)	78 %	161 %	175 %	186 %	158 %

(a)	Based on average shares outstanding.
(b)	Distributions for annual periods determined in accordance with U.S. federal income tax regulations.
(c)	Amount is greater than $(0.005) per share.
(d)	Where applicable, assumes the reinvestment of distributions.
(e)	Includes payment from an affiliate, which had no impact on the Fund’s total return.
(f)	Includes the Fund’s share of the Master Portfolio’s allocated net expenses and/or net investment income.
(g)	Excludes fees and expenses incurred indirectly as a result of investments in underlying funds.
(h)	Includes mortgage dollar roll transactions (“MDRs”). Additional information regarding portfolio turnover rate is as follows:

	Year Ended 12/31/23	Year Ended 12/31/22	Year Ended 12/31/21	Year Ended 12/31/20	Year Ended 12/31/19

Portfolio turnover rate (excluding MDRs)	44 %	93 %	89 %	101 %	97 %

Financial Highlights (concluded)

	iShares U.S. Aggregate Bond Index Fund
	Investor A
	Year Ended 12/31/23	Year Ended 12/31/22	Year Ended 12/31/21	Year Ended 12/31/20	Year Ended 12/31/19
(For a share outstanding throughout each period)	Year Ended 12/31/23	Year Ended 12/31/22	Year Ended 12/31/21	Year Ended 12/31/20	Year Ended 12/31/19
Net asset value, beginning of year	$8.89	$10.48	$10.89	$10.39	$9.84
Net investment income(a)	0.26	0.18	0.16	0.21	0.26
Net realized and unrealized gain (loss)	0.21	(1.57 )	(0.39 )	0.54	0.56
Net increase (decrease) from investment operations	0.47	(1.39 )	(0.23 )	0.75	0.82
Distributions(b)
From net investment income	(0.26 )	(0.20 )	(0.18 )	(0.23 )	(0.27 )
From net realized gain	—	—	(0.00 )(c)	(0.02 )	—
Return of capital	(0.00 )(c)	—	(0.00 )(c)	—	—
Total distributions	(0.26 )	(0.20 )	(0.18 )	(0.25 )	(0.27 )
Net asset value, end of year	$9.10	$8.89	$10.48	$10.89	$10.39
Total Return(d)
Based on net asset value	5.39 %	(13.31 )%	(2.09 )%	7.33 %	8.39 %(e)
Ratios to Average Net Assets(f)(g)
Total expenses	0.35 %	0.35 %	0.34 %	0.34 %	0.35 %
Total expenses after fees waived and/or reimbursed	0.35 %	0.35 %	0.34 %	0.34 %	0.35 %
Net investment income	2.93 %	1.90 %	1.46 %	1.98 %	2.61 %
Supplemental Data
Net assets, end of year (000)	$131,280	$96,331	$144,418	$161,624	$121,421
Portfolio turnover rate of the Master Portfolio(h)	78 %	161 %	175 %	186 %	158 %

(a)	Based on average shares outstanding.
(b)	Distributions for annual periods determined in accordance with U.S. federal income tax regulations.
(c)	Amount is greater than $(0.005) per share.
(d)	Where applicable, assumes the reinvestment of distributions.
(e)	Includes payment from an affiliate, which had no impact on the Fund’s total return.
(f)	Includes the Fund’s share of the Master Portfolio’s allocated net expenses and/or net investment income.
(g)	Excludes fees and expenses incurred indirectly as a result of investments in underlying funds.
(h)	Includes mortgage dollar roll transactions (“MDRs”). Additional information regarding portfolio turnover rate is as follows:

	Year Ended 12/31/23	Year Ended 12/31/22	Year Ended 12/31/21	Year Ended 12/31/20	Year Ended 12/31/19

Portfolio turnover rate (excluding MDRs)	44 %	93 %	89 %	101 %	97 %

General Information

Shareholder Documents

Electronic Access to Annual Reports, Semi-Annual Reports and Prospectuses

Electronic copies of most financial reports and prospectuses are available on BlackRock’s website. Shareholders can sign up for e-mail notifications of annual and semi-annual reports and prospectuses by enrolling in the Fund’s electronic delivery program. To enroll:

Shareholders Who Hold Accounts with Investment Advisers, Banks or Brokerages: Please contact your Financial Intermediary. Please note that not all investment advisers, banks or brokerages may offer this service.

Shareholders Who Hold Accounts Directly With BlackRock:

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Access the BlackRock website at http://www.blackrock.com/edelivery; and

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Log into your account.

Delivery of Shareholder Documents

The Fund delivers only one copy of shareholder documents, including prospectuses, shareholder reports and proxy statements, to shareholders with multiple accounts at the same address. This practice is known as “householding” and is intended to eliminate duplicate mailings and reduce expenses. Mailings of your shareholder documents may be householded indefinitely unless you instruct us otherwise. If you do not want the mailing of these documents to be combined with those for other members of your household, please contact the Fund at (800) 441-7762.

Certain Fund Policies

Anti-Money Laundering Requirements

The Fund is subject to the USA PATRIOT Act (the “Patriot Act”). The Patriot Act is intended to prevent the use of the U.S. financial system in furtherance of money laundering, terrorism or other illicit activities. Pursuant to requirements under the Patriot Act, the Fund is required to obtain sufficient information from shareholders to enable it to form a reasonable belief that it knows the true identity of its shareholders. This information will be used to verify the identity of investors or, in some cases, the status of Financial Intermediaries. Such information may be verified using third-party sources. This information will be used only for compliance with the Patriot Act or other applicable laws, regulations and rules in connection with money laundering, terrorism or economic sanctions.

The Fund reserves the right to reject purchase orders from persons who have not submitted information sufficient to allow the Fund to verify their identity. The Fund also reserves the right to redeem any amounts in the Fund from persons whose identity it is unable to verify on a timely basis. It is the Fund’s policy to cooperate fully with appropriate regulators in any investigations conducted with respect to potential money laundering, terrorism or other illicit activities.

BlackRock Privacy Principles

BlackRock is committed to maintaining the privacy of its current and former fund investors and individual clients (collectively, “Clients”) and to safeguarding their non-public personal information. The following information is provided to help you understand what personal information BlackRock collects, how we protect that information and why in certain cases we share such information with select parties.

If you are located in a jurisdiction where specific laws, rules or regulations require BlackRock to provide you with additional or different privacy-related rights beyond what is set forth below, then BlackRock will comply with those specific laws, rules or regulations.

BlackRock obtains or verifies personal non-public information from and about you from different sources, including the following: (i) information we receive from you or, if applicable, your Financial Intermediary, on applications, forms or other documents; (ii) information about your transactions with us, our affiliates, or others; (iii) information we receive from a consumer reporting agency; and (iv) from visits to our website.

BlackRock does not sell or disclose to non-affiliated third parties any non-public personal information about its Clients, except as permitted by law, or as is necessary to respond to regulatory requests or to service Client accounts. These non-affiliated third parties are required to protect the confidentiality and security of this information and to use it only for its intended purpose.

We may share information with our affiliates to service your account or to provide you with information about other BlackRock products or services that may be of interest to you. In addition, BlackRock restricts access to non-public personal information about its Clients to those BlackRock employees with a legitimate business need for the information. BlackRock maintains physical, electronic and procedural safeguards that are designed to protect the non-public personal information of its Clients, including procedures relating to the proper storage and disposal of such information.

Statement of Additional Information

If you would like further information about the Fund, including how it invests, please see the SAI.

For a discussion of the Fund’s policies and procedures regarding the selective disclosure of its portfolio holdings, please see the SAI. The Fund makes its top ten holdings available on a monthly basis at www.blackrock.com generally within 5 business days after the end of the month to which the information applies.

Glossary

This glossary contains an explanation of some of the common terms used in this prospectus. For additional information about the Fund, please see the SAI.

Acquired Fund Fees and Expenses — the Fund’s pro rata share of the fees and expenses incurred indirectly by the Fund as a result of investing in other investment companies.

Administration Fee — a fee paid to the administrator for providing administrative services to a Fund.

Annual Fund Operating Expenses — expenses that cover the costs of operating the Fund.

Distribution Fees — fees used to support the Fund’s marketing and distribution efforts, such as compensating Financial Intermediaries, advertising and promotion.

Management Fee — a fee paid to BFA for managing the Master Portfolio.

Other Expenses — include administration, accounting, transfer agency, custody, professional and registration fees.

Bloomberg U.S. Aggregate Bond Index — a broad-based flagship benchmark that measures the investment grade, U.S. dollar-denominated, fixed-rate taxable bond market. The index includes U.S. Treasuries, government-related and corporate securities, mortgage-backed securities (agency fixed-rate pass-throughs), asset-backed securities and commercial mortgage-backed securities (agency and non-agency).

Service Fees — fees used to compensate Financial Intermediaries for certain shareholder servicing activities.

Additional Information

For more information:
This prospectus contains important information you should know before investing, including information about risks. Please read it before you invest and keep it for future reference. More information about the Fund is available at no charge upon request. This information includes:

Annual/Semi-Annual Reports
These reports contain additional information about the Fund’s investments. The annual report describes the Fund’s performance, lists portfolio holdings, and discusses recent market conditions, economic trends and Fund investment strategies that significantly affected the Fund’s performance for the last fiscal year.

Statement of Additional Information
A Statement of Additional Information (“SAI”), dated April 29, 2024, has been filed with the Securities and Exchange Commission (the “SEC”). The SAI, which includes additional information about the Fund, may be obtained free of charge, along with the Fund’s annual and semi-annual reports, by calling (800) 441-7762. The SAI, as amended and/or supplemented from time to time, is incorporated by reference into this prospectus.

Questions
If you have any questions about the Fund, please call: (800) 441-7762 (toll-free) 8:00 a.m. to 6:00 p.m. (Eastern time) on any business day.

World Wide Web
General Fund information and specific Fund performance, including the SAI and annual/semi-annual reports, can be accessed free of charge at www.blackrock.com/prospectus. Mutual fund prospectuses and literature can also be requested via this website.

Written Correspondence
BlackRock Funds III
P.O. Box 534429
Pittsburgh, Pennsylvania 15253-4429

Overnight Mail
BlackRock Funds III
Attention: 534429
500 Ross Street 154-0520
Pittsburgh, Pennsylvania 15262

Portfolio Characteristics and Holdings
A description of the Fund’s policies and procedures related to disclosure of portfolio characteristics and holdings is available in the SAI.

For information about portfolio holdings and characteristics, BlackRock fund shareholders and prospective investors may call (800) 882-0052.

Securities and Exchange Commission
You may also view and copy public information about the Fund, including the SAI, by visiting the EDGAR database on the SEC’s website (http://www.sec.gov). Copies of this information can be obtained, for a duplicating fee, by electronic request at the following e-mail address: [email protected].

You should rely only on the information contained in this prospectus. No one is authorized to provide you with information that is different from information contained in this prospectus.

The SEC has not approved or disapproved these securities or passed upon the adequacy of this prospectus. Any representation to the contrary is a criminal offense.

INVESTMENT COMPANY ACT FILE # 811-07332

PRO-USABI-0424

For More Information	Fund and Service Providers	Inside Back Cover
	Additional Information	Back Cover

Fund Overview	Key facts and details about the Fund, including investment objective, principal investment strategies, principal risk factors, fee and expense information and historical performance information
	Investment Objective	3
	Fees and Expenses of the Fund	3
	Principal Investment Strategies of the Fund	4
	Principal Risks of Investing in the Fund	4
	Performance Information	6
	Investment Adviser	7
	Portfolio Managers	7
	Purchase and Sale of Fund Shares	7
	Tax Information	8
	Payments to Broker/Dealers and Other Financial Intermediaries	8

Details About the Fund	Information about how the Fund invests, including investment objective, investment processes, principal strategies and risk factors
	Details About the Fund’s Investment Strategies	10
	Investment Risks	11

Account Information	Information about account services, sales charges and waivers, shareholder transactions, and distribution and other payments
	How to Choose the Share Class that Best Suits Your Needs	20
	Details About the Share Classes	23
	Distribution and Shareholder Servicing Payments	24
	How to Buy, Sell, Exchange and Transfer Shares	25
	Account Services and Privileges	30
	Fund’s Rights	31
	Participation in Fee-Based Programs	32
	Short-Term Trading Policy	32
	Master/Feeder Mutual Fund Structure	33

Management of the Fund	Information about BlackRock Fund Advisors and the Portfolio Managers
	Investment Adviser	34
	Portfolio Managers	34
	Administrative Services	35
	Conflicts of Interest	35
	Valuation of Fund Investments	36
	Dividends, Distributions and Taxes	37