Summary Prospectus by Investment Company (Form 497K)

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iShares Russell 1000 Large-Cap Index Fund

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The fees and expenses shown in the table above and the example that follows include both the expenses of the Fund and its share of the allocated expenses of Large Cap Index Master Portfolio (the "Master Portfolio"), a series of Master Investment Portfolio ("MIP"). The management fees are paid by the Master Portfolio.

2

As described in the "Management of the Funds" section of the Fund's prospectus beginning on page 45, BFA, the investment adviser for the Master Portfolio, 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, 2027. In addition, 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 money market funds managed by BFA or its affiliates, through June 30, 2027. The contractual agreements 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.

3

As described in the "Management of the Funds" section of the Fund's prospectus beginning on page 45, BFA and BlackRock Advisors, LLC ("BAL"), the administrator for the Fund, have contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements of the Fund (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.38% (for Investor A Shares) and 0.13% (for Institutional Shares) of average daily net assets through June 30, 2027. This contractual agreement may be terminated upon 90 days' notice by a majority of the non-interested trustees of the Trust or of MIP, as applicable, or by a vote of a majority of the outstanding voting securities of the Fund or the Master Portfolio, as applicable.

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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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Equity Securities Risk - Stock markets are volatile. The price of equity securities fluctuates based on changes in a company's financial condition and overall market and economic conditions.

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Information Technology Sector Risk - IT companies face intense competition, both domestically and internationally, which may have an adverse effect on their profit margins. Like other technology companies, IT companies may have limited product lines, markets, financial resources or personnel. The products of IT companies may face obsolescence due to rapid technological developments, frequent new product introduction, unpredictable changes in growth rates and competition for the services of qualified personnel. Companies in the IT sector are heavily dependent on patent and intellectual property rights. The loss or impairment of these rights may adversely

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Futures Risk - The Fund's use of futures may reduce the Fund's returns. In these transactions, the Fund is subject to liquidity risk and correlation risk (i.e., that fluctuations in a future's value may not correlate with the change in market value of the instruments held by the Fund).

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Index-Related Risk - The index provider may rely on various sources of information to assess the criteria of components of the underlying index, including information that may be based on assumptions and estimates. Neither the Fund nor BlackRock can offer assurances that the index provider's methodology or sources of information will provide an accurate assessment of included components. Errors in index data, index computations or the construction of the underlying index in accordance with its methodology may occur, and the index provider may not identify or correct them promptly 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.

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Large-Capitalization Companies Risk - Large-capitalization companies may be less able than smaller-capitalization companies to adapt to changing market conditions and competitive challenges. Large-capitalization companies may be more mature and subject to more limited growth potential compared with smaller-capitalization companies. The performance of large-capitalization companies could trail the overall performance of the broader securities markets. Fund performance depends on the performance of individual securities to which the Fund has exposure. Changes in the financial condition or credit rating of an issuer of those securities may cause the value of the securities to decline.

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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.

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Mid Cap Securities Risk - The securities of mid cap companies generally trade in lower volumes and are generally subject to greater and less predictable price changes than the securities of larger capitalization companies.

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Non-Diversification Risk - To the extent the Fund is non-diversified, the Fund may invest a large percentage of its assets in securities or other instruments representing a small number of issuers or counterparties and thus may be more susceptible to the risks associated with these particular issuers or counterparties. As a result, the Fund's performance may depend to a greater extent on the performance of a small number of issuers or counterparties, which may lead to more volatility in the Fund's net asset value.

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Operational and Technology Risks - The Fund is directly and indirectly susceptible to operational and technology risks, including those related to human errors, processing errors, communication errors, systems failures, cybersecurity incidents, and the use of artificial intelligence and machine learning ("AI"), which may result in losses for the Fund and its shareholders or may impair the Fund's operations. While the Fund's service providers are required to have appropriate operational, information security and cybersecurity risk management policies and procedures, their methods of risk management may differ from those of the Fund. Operational and technology risks for the issuers in which the Fund invests could also result in material adverse consequences for such issuers and may cause the Fund's investments in such issuers to lose value.

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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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Preferred Securities Risk - Preferred securities may pay fixed or adjustable rates of return. Preferred securities are subject to issuer-specific and market risks applicable generally to equity securities. In addition, a company's

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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.

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Tracking Error Risk - The Fund is subject to the risk of "tracking error," which is the divergence of the Fund's performance from that of the Underlying Index. Tracking error may occur due to a number of factors, including differences between the securities and other assets held in the Fund's portfolio and those included in the Underlying Index; differences in the timing and methodologies used to value securities and other assets; transaction costs and other expenses incurred by the Fund that the Underlying Index does not incur; the Fund's holding of uninvested cash; differences in the timing of the accrual or the valuation of dividends or interest received by the Fund or distributions paid to Fund shareholders; tax gains or losses; differences between the amount and/or timing of withholding taxes on dividends reflected in the Underlying Index from the Fund's obligation, if any, for foreign withholding taxes; the requirements for the Fund to maintain pass-through tax treatment; portfolio transactions carried out to minimize the distribution of capital gains to shareholders; changes to the Underlying Index, such as during a rebalancing or reconstitution; and impacts to the Fund of complying with certain regulatory requirements or limits. A fund that tracks an index composed of a large number of securities or other assets may experience greater tracking error than a fund that tracks a more narrow index. Tracking error risk may be heightened during times of increased market volatility or other unusual market conditions.