MainStay Epoch U.S. Small Cap Fund		Summary Prospectus February 29, 2016
Class/Ticker	A MOPAXInvestor MOINXB MOTBXC MOPCXI MOPIXR1 MOPRXR2 MOTRX R3 MOVRX

To Statutory ProspectusTo Statement of Additional Information

Before you invest, you may want to review the Fund's Prospectus, which contains more information about the Fund and its risks. You can find the Fund's Prospectus and other information about the Fund by going online to mainstayinvestments.com/documents, by calling 800-MAINSTAY (624-6782) or by sending an e-mail to MainStayShareholderServices@nylim.com. The Fund's Prospectus and Statement of Additional Information, both dated February 29, 2016, as may be amended from time to time, are incorporated by reference into this Summary Prospectus.

Investment Objective

The Fund seeks long-term capital appreciation by investing primarily in securities of small-cap companies.

Fees and Expenses of the Fund

The table below describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the MainStay Funds. This amount may vary depending on the MainStay Fund in which you invest. More information about these and other discounts is available from your financial professional and in the "Information on Sales Charges" section starting on page 100 of the Prospectus and in the "Alternative Sales Arrangements" section on page 111 of the Statement of Additional Information.

		Class A		Investor  Class		Class B		Class C		Class I		Class R1		Class R2		Class R3
Shareholder Fees (fees paid directly from your investment)
	Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	5.50	%	5.50	%	None		None		None		None		None		None
	Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	None	1	None	1	5.00	%	1.00	%	None		None		None		None
Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
	Management Fees (as an annual percentage of the Fund's average daily net assets)2	0.85	%	0.85	%	0.85	%	0.85	%	0.85	%	0.85	%	0.85	%	0.85	%
	Distribution and/or Service (12b-1) Fees	0.25	%	0.25	%	1.00	%	1.00	%	None		None		0.25	%	0.50	%
	Other Expenses	0.15	%	0.41	%	0.41	%	0.41	%	0.15	%	0.25	%	0.25	%	0.25	%3
	Total Annual Fund Operating Expenses	1.25	%	1.51	%	2.26	%	2.26	%	1.00	%	1.10	%	1.35	%	1.60	%

1.A contingent deferred sales charge of 1.00% may be imposed on certain redemptions made within one year of the date of purchase on shares that were purchased without an initial sales charge.

2.The management fee is as follows: 0.85% on assets up to $1 billion; and 0.80% on assets in excess of $1 billion.

3.Based on estimated amounts for the current fiscal year.

Example

The Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods (except as indicated with respect to Class B and Class C shares). The Example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain the same. The Example reflects the contractual fee waiver and/or expense reimbursement arrangement, if applicable, for the current duration of the arrangement only. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expenses After	Class A	Investor	Class B		Class C		Class I	Class R1	Class R2	Class R3
		Class	Assuming no redemption	Assuming redemption at end of period	Assuming no redemption	Assuming redemption at end of period
1 Year	$       670	$       695	$       229	$       729	$       229	$       329	$       102	$       112	$       137	$       163
3 Years	$       925	$    1,001	$       706	$    1,006	$       706	$       706	$       318	$       350	$       428	$       505
5 Years	$    1,199	$    1,328	$    1,210	$    1,410	$    1,210	$    1,210	$       552	$       606	$       739	$       871
10 Years	$    1,978	$    2,252	$    2,407	$    2,407	$    2,595	$    2,595	$    1,225	$    1,340	$    1,624	$    1,900

Portfolio Turnover

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover may indicate higher transaction costs and may result in higher taxes when Fund 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 Fund's portfolio turnover rate was 39% of the average value of its portfolio.

(NYLIM) NL246 MSEUSC01-02/16

Principal Investment Strategies

The Fund normally invests at least 80% of its assets (net assets plus any borrowings for investment purposes) in equity securities of U.S. companies with market capitalizations at the time of investment of $6 billion or less, which include common stocks, securities convertible into common stock and exchange traded funds ("ETFs") whose underlying securities are issued by small capitalization companies. The Fund may also invest in mid-cap stocks. Securities of U.S. companies are those traded primarily in the U.S. securities markets.

Investment Process: Epoch Investment Partners, Inc., the Fund's Subadvisor, invests primarily in companies that generate increasing levels of free cash flow and have managements that allocate it effectively to create shareholder value.

The security selection process focuses on free-cash-flow analytics as opposed to traditional accounting-based metrics. The Subadvisor seeks to identify companies with a consistent, straightforward ability to both generate free cash flow and to intelligently allocate it among internal reinvestment opportunities, acquisitions, dividends, share repurchases and/or debt reduction.

The Subadvisor may sell or reduce a position in a security when it believes its investment objectives have been met or when the security is deemed less attractive relative to another security on a return/risk basis. The Subadvisor may sell or reduce a position in a security if it sees the investment thesis failing to materialize.

Principal Risks

Loss of Money Risk: Before considering an investment in the Fund, you should understand that you could lose money.

Market Changes Risk: The value of the Fund's investments may fluctuate because of changes in the markets in which the Fund invests, which could cause the Fund to underperform other funds with similar objectives. Changes in these markets may be rapid and unpredictable. From time to time, markets may experience periods of stress for potentially prolonged periods that may result in (i) increased market volatility; (ii) reduced market liquidity; and (iii) increased redemptions. Such conditions may add significantly to the risk of volatility in the net asset value of the Fund's shares.

Portfolio Management Risk: The investment strategies, practices and risk analysis used by the Subadvisor may not produce the desired results. In addition, the Fund may not achieve its investment objective if the Subadvisor takes temporary positions in response to unusual or adverse market, economic or political conditions, or other unusual or abnormal circumstances.

Selection Risk: The investments selected by the Fund's Subadvisor may underperform the market or other investments.

Equity Securities Risk: Investments in common stocks and other equity securities are particularly subject to the risk of changing economic, stock market, industry and company conditions and the risks inherent in the portfolio managers' ability to anticipate such changes that can adversely affect the value of the Fund's holdings. Opportunity for greater gain often comes with greater risk of loss.

Growth Stock Risk: If growth companies do not increase their earnings at a rate expected by investors, the market price of the stock may decline significantly, even if earnings show an absolute increase. Growth company stocks also typically lack the dividend yield that can cushion stock prices in market downturns.

Value Stock Risk: Value stocks may never reach what the Subadvisor believes is their full value or they may go down in value. In addition, different types of stocks tend to shift in and out of favor depending on market and economic conditions, and therefore the Fund's performance may be lower or higher than that of funds that invest in other types of equity securities.

Exchange-Traded Fund Risk: The risks of owning an ETF generally reflect the risks of owning the underlying securities they are designed to track, although lack of liquidity in an ETF could result in it being more volatile than the underlying portfolio of securities. Disruptions in the markets for the securities underlying ETFs purchased or sold by the Fund could result in losses on the Fund's investment in ETFs. ETFs also have management fees that increase their costs versus the costs of owning the underlying securities directly.

Small-Cap and Mid-Cap Stock Risk: Stocks of small-cap and mid-cap companies may be subject to greater price volatility, significantly lower trading volumes, cyclical, static or moderate growth prospects and greater spreads between their bid and ask prices than stocks of larger companies. Because these businesses frequently rely on narrower product lines and niche markets, they can suffer isolated setbacks. Smaller capitalization companies may be more vulnerable to adverse business or market developments.

Convertible Securities Risk: Convertible securities may be subordinate to other securities. In part, the total return for a convertible security depends upon the performance of the underlying stock into which it can be converted. Also, issuers of convertible securities are often not as strong financially as those issuing securities with higher credit ratings, are more likely to encounter financial difficulties and typically are more vulnerable to changes in the economy, such as a recession or a sustained period of rising interest rates, which could affect their ability to make interest and principal payments. If an issuer stops making interest and/or principal payments, the Fund could lose its entire investment.

Foreign Securities Risk: Investments in foreign securities may be riskier than investments in U.S. securities. Differences between U.S. and foreign regulatory regimes and securities markets, including less stringent investor protections and disclosure standards of some foreign markets, less liquid trading markets and political and economic developments in foreign countries, may affect the value of the Fund's investments in foreign securities. Foreign securities may also subject the Fund's investments to changes in currency rates. These risks may be greater with respect to securities of companies that conduct their business activities in emerging markets or whose securities are traded principally in emerging markets.

Past Performance

The following bar chart and tables indicate some of the risks of investing in the Fund. The bar chart shows you how the Fund's calendar year performance has varied over the last ten years. Sales loads are not reflected in the bar chart. If they were, returns would be less than those shown. The average annual total returns table shows how the Fund's average annual total returns (before and after taxes) for the one-, five- and ten-year periods compare to those of a broad-based securities market index. The Fund has selected the Russell 2500^TM Index as its primary benchmark. The Russell 2500^TM Index measures the

2

performance of the small to mid-cap segment of the U.S. equity universe, commonly referred to as "smid" cap. The Russell 2500^TM Index is a subset of the Russell 3000^® Index. It includes approximately 2,500 of the smallest securities based on a combination of their market cap and current index membership.

Performance data for the classes varies based on differences in their fee and expense structures. Performance figures for Investor Class shares, first offered on February 28, 2008, include the historical performance of Class A shares through February 27, 2008. Performance figures for Class R1 and Class R2 shares, first offered on July 31, 2012, include the historical performance of Class I shares through July 30, 2012. Performance figures for Class R3 shares, first offered on February 29, 2016, include the historical performance of Class I shares through December 31, 2015. The performance for these newer share classes is adjusted for differences in certain fees and expenses. Unadjusted, the performance shown for these newer share classes would likely have been different. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Please visit mainstayinvestments.com for more recent performance information.

Annual Returns, Class I Shares

(by calendar year 2006-2015)

Best Quarter
2Q/09	38.29	%
Worst Quarter
4Q/08	-25.23	%

Average Annual Total Returns (for the periods ended December 31, 2015)

	1 Year		5 Years		10 Years
Return Before Taxes
Class A	-9.40	%	7.56	%	4.50	%
Investor Class	-9.64	%	7.24	%	4.31	%
Class B	-9.82	%	7.36	%	4.12	%
Class C	-6.02	%	7.66	%	4.12	%
Class I	-3.89	%	9.05	%	5.44	%
Class R1	-3.94	%	8.95	%	5.34	%
Class R2	-4.18	%	8.68	%	5.08	%
Class R3	-4.46	%	8.40	%	4.81	%
Return After Taxes on Distributions
Class I	-3.97	%	8.98	%	5.11	%
Return After Taxes on Distributions and Sale of Fund Shares
Class I	-2.13	%	7.15	%	4.30	%
Russell 2500™ Index (reflects no deductions for fees, expenses, or taxes)	-2.90	%	10.32	%	7.56	%

After-tax returns are calculated using the highest individual federal marginal income tax rates in effect at the time of each distribution or capital gain or upon the sale of fund shares, and do not reflect the impact of state and local taxes. In some cases, the return after taxes may exceed the return before taxes due to an assumed tax benefit from any losses on a sale of shares at the end of the measurement period. Actual after-tax returns depend on your tax situation and may differ from those shown. After-tax returns are not relevant if you hold your shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns shown are for Class I shares. After-tax returns for the other share classes may vary.

Management

New York Life Investment Management LLC serves as the Fund's Manager. Epoch Investment Partners, Inc. serves as the Fund's Subadvisor.

Subadvisor	Portfolio Managers	Service Date
Epoch Investment Partners, Inc.	David Pearl, Executive Vice President & Co-Chief Investment Officer	Since 2009
	Michael Welhoelter, Managing Director	Since 2009
	Michael J. Caputo, Managing Director & Senior Research Analyst	Since February 2016

How to Purchase and Sell Shares

You may purchase or sell shares of the Fund on any day the Fund is open for business by contacting your financial adviser or financial intermediary firm, or by contacting the Fund by telephone at 800-MAINSTAY (624-6782), by mail at MainStay Funds, P.O. Box 8401, Boston, MA 02266-8401 or by accessing

3

our website at mainstayinvestments.com. Generally, an initial investment minimum of $1,000 applies if you invest in Investor Class, Class B, or Class C shares, $25,000 for Class A shares and $5,000,000 for individual investors in Class I shares investing directly (i) with the Fund; or (ii) through certain private banks and trust companies that have an agreement with NYLIFE Distributors LLC, the Fund’s principal underwriter and distributor, or its affiliates. A subsequent investment minimum of $50 applies to investments in Investor Class, Class B and Class C shares. However, for Investor Class, Class B, or Class C shares purchased through AutoInvest, MainStay’s systematic investment plan, a $500 initial investment minimum and $50 minimum for subsequent purchases applies. Class R1 shares, Class R2 shares, Class R3 shares and institutional shareholders in Class I shares have no initial or subsequent investment minimums.

Tax Information

The Fund's distributions are generally taxable to you as ordinary income, capital gains, or a combination of the two, unless you are investing through a tax-deferred arrangement, such as a 401(k) plan or an individual retirement account.

Compensation to Financial Intermediary Firms

If you purchase Fund shares through a financial intermediary firm (such as a broker/dealer or bank), the Fund and its related companies may pay the intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the financial intermediary firm or your financial adviser to recommend the Fund over another investment. Ask your financial adviser or visit your financial intermediary firm's website for more information. For additional information about compensation to financial intermediaries, please see the section entitled "Compensation to Financial Intermediary Firms" in the "Shareholder Guide" section starting on page 106 of the Prospectus.

To Statutory ProspectusTo Statement of Additional Information

4