MAINSTAY INSTITUTIONAL FUNDS INC
NSAR-A, 1999-08-30
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<PAGE>      PAGE  1
000 A000000 06/30/1999
000 C000000 868307
000 D000000 N
000 E000000 NF
000 F000000 Y
000 G000000 N
000 H000000 N
000 I000000 6.1
000 J000000 A
001 A000000 MAINSTAY INSTITUTIONAL FUNDS INC.
001 B000000 811-6175
001 C000000 2125768170
002 A000000 51 MADISON AVENUE
002 B000000 NEW YORK
002 C000000 NY
002 D010000 10010
003  000000 N
004  000000 N
005  000000 N
006  000000 N
007 A000000 Y
007 B000000 11
007 C010100  1
007 C020100 MONEY MARKET FUND
007 C030100 N
007 C010200  2
007 C020200 SHORT-TERM BOND FUND
007 C030200 N
007 C010300  3
007 C020300 BOND FUND
007 C030300 N
007 C010400  4
007 C020400 INDEXED BOND FUND
007 C030400 N
007 C010500  5
007 C020500 ASSET MANAGER FUND
007 C030500 N
007 C010600  6
007 C020600 VALUE EQUITY FUND
007 C030600 N
007 C010700  7
007 C020700 GROWTH EQUITY FUND
007 C030700 N
007 C010800  8
007 C020800 INDEXED EQUITY FUND
007 C030800 N
007 C010900  9
007 C020900 EAFE INDEX FUND
007 C030900 N
007 C011000 10
007 C021000 INTERNATIONAL BOND FUND
<PAGE>      PAGE  2
007 C031000 N
007 C011100 11
007 C021100 INTERNATIONAL EQUITY FUND
007 C031100 N
007 C011200 12
007 C011300 13
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007 C011600 16
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007 C011800 18
007 C011900 19
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<PAGE>      PAGE  3
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011 C01AA01 PARSIPPANY
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012 A00AA01 MAINSTAY SHAREHOLDER SERIVCES INC.
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013 A00AA01 PRICEWATERHOUSE COOPERS LLP
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013 B03AA01 10022
014 A00AA01 NYLIFE SECURITIES, INC.
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014 A00AA02 NYLIFE DISTRIBUTORS INC.
014 B00AA02 8-46655
015 A00AA01 BANK OF NEW YORK
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020 A000006 MORGAN (J.P.) SECURITES, INC.
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020 A000007 MERRILL LYNCH PIERCE, FENNER & SMITH
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<PAGE>      PAGE  4
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<PAGE>      PAGE  5
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<PAGE>      PAGE  6
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<PAGE>      PAGE  7
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<PAGE>      PAGE  8
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<PAGE>      PAGE  9
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<PAGE>      PAGE  10
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008 D030201 10019
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<PAGE>      PAGE  11
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<PAGE>      PAGE  12
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<PAGE>      PAGE  13
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008 D030301 10019
008 A000302 MAINSTAY MANAGEMENT, INC.
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<PAGE>      PAGE  15
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<PAGE>      PAGE  17
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<PAGE>      PAGE  20
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<PAGE>      PAGE  21
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<PAGE>      PAGE  22
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<PAGE>      PAGE  26
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<PAGE>      PAGE  31
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<PAGE>      PAGE  32
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008 A000701 MACKAY-SHIELDS FINANCIAL CORPORATION
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<PAGE>      PAGE  35
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<PAGE>      PAGE  37
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<PAGE>      PAGE  38
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048 J010800        0
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048 K010800        0
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050  000800 N
051  000800 N
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053 B000800 Y
053 C000800 N
058 A000800 N
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<PAGE>      PAGE  39
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066 C000800 Y
066 D000800 N
066 E000800 N
066 F000800 N
066 G000800 N
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068 A000800 N
068 B000800 N
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070 A010800 Y
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070 C010800 N
070 C020800 N
070 D010800 Y
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070 E010800 N
070 E020800 N
070 F010800 Y
070 F020800 Y
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070 H010800 Y
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070 I010800 N
070 I020800 N
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070 J020800 N
070 K010800 Y
070 K020800 N
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070 L020800 N
070 M010800 Y
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070 O020800 N
070 P010800 N
070 P020800 N
070 Q010800 Y
070 Q020800 Y
070 R010800 N
070 R020800 N
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<PAGE>      PAGE  40
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<PAGE>      PAGE  41
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008 A000901 MONITOR CAPITAL ADVISORS, INC.
008 B000901 S
008 C000901 801-34412
008 D010901 PRINCETON
008 D020901 NJ
008 D030901 08540
008 D040901 6242
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008 B000902 A
008 C000902 801-54912
008 D010902 PARSIPPANY
008 D020902 NJ
008 D030902 07054
024  000900 N
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<PAGE>      PAGE  42
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045  000900 Y
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049  000900 N
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051  000900 N
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<PAGE>      PAGE  43
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<PAGE>      PAGE  44
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<PAGE>      PAGE  45
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008 B001001 S
008 C001001 801-5594
008 D011001 NEW YORK
008 D021001 NY
008 D031001 10019
008 A001002 MAINSTAY MANAGEMENT, INC.
008 B001002 A
008 C001002 801-54912
008 D011002 PARSIPPANY
008 D021002 NJ
008 D031002 07054
024  001000 N
<PAGE>      PAGE  46
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<PAGE>      PAGE  47
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<PAGE>      PAGE  48
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<PAGE>      PAGE  49
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<PAGE>      PAGE  50
008 A001101 MACKAY-SHIELDS FINANCIAL CORPORATION
008 B001101 S
008 C001101 801-5594
008 D011101 NEW YORK
008 D021101 NY
008 D031101 10019
008 A001102 MAINSTAY MANAGEMENT, INC.
008 B001102 A
008 C001102 801-54912
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008 D031102 07054
024  001100 N
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<PAGE>      PAGE  51
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<PAGE>      PAGE  52
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<PAGE>      PAGE  53
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<PAGE>      PAGE  54
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SIGNATURE   MICHAEL J. MCDONOUGH
TITLE       ATTORNEY


WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 1
   <NAME> SWEEP SHARES
<MULTIPLIER> 1

<S>                             <C>
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<EQUALIZATION>                                       0
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<NUMBER-OF-SHARES-REDEEMED>               (58,824,926)
<SHARES-REINVESTED>                          1,715,458
<NET-CHANGE-IN-ASSETS>                     133,206,120
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                     (27,255)
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                        (162,000)
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                            (1,279,000)
<AVERAGE-NET-ASSETS>                       325,730,000
<PER-SHARE-NAV-BEGIN>                            1.000
<PER-SHARE-NII>                                  0.020
<PER-SHARE-GAIN-APPREC>                          0.000
<PER-SHARE-DIVIDEND>                           (0.020)
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                              1.000
<EXPENSE-RATIO>                                  1.000


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 1
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
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<INVESTMENTS-AT-VALUE>                     371,258,693
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<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             372,493,782
<PAYABLE-FOR-SECURITIES>                             0
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                    1,695,261
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<SENIOR-EQUITY>                                      0
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<SHARES-COMMON-STOCK>                        5,448,898
<SHARES-COMMON-PRIOR>                      103,764,649
<ACCUMULATED-NII-CURRENT>                            0
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                       (21,906)
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                             0
<NET-ASSETS>                               370,798,521
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            8,186,340
<OTHER-INCOME>                                       0
<EXPENSES-NET>                             (1,098,912)
<NET-INVESTMENT-INCOME>                      7,087,428
<REALIZED-GAINS-CURRENT>                         5,349
<APPREC-INCREASE-CURRENT>                            0
<NET-CHANGE-FROM-OPS>                        7,092,777
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                    (403,178)
<DISTRIBUTIONS-OF-GAINS>                             0
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<NUMBER-OF-SHARES-SOLD>                      9,786,504
<NUMBER-OF-SHARES-REDEEMED>              (108,859,634)
<SHARES-REINVESTED>                            757,379
<NET-CHANGE-IN-ASSETS>                    (91,626,152)
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                     (27,255)
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                        (162,000)
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                            (1,279,000)
<AVERAGE-NET-ASSETS>                       325,730,000
<PER-SHARE-NAV-BEGIN>                            1.000
<PER-SHARE-NII>                                  0.020
<PER-SHARE-GAIN-APPREC>                          0.000
<PER-SHARE-DIVIDEND>                           (0.020)
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                              1.000
<EXPENSE-RATIO>                                 0.7500


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 1
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      371,258,693
<INVESTMENTS-AT-VALUE>                     371,258,693
<RECEIVABLES>                                1,221,006
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<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             372,493,782
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<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                    1,695,261
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<SENIOR-EQUITY>                                      0
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<ACCUMULATED-NII-CURRENT>                            0
<OVERDISTRIBUTION-NII>                               0
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<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                             0
<NET-ASSETS>                               370,798,521
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            8,186,340
<OTHER-INCOME>                                       0
<EXPENSES-NET>                             (1,098,912)
<NET-INVESTMENT-INCOME>                      7,087,428
<REALIZED-GAINS-CURRENT>                         5,349
<APPREC-INCREASE-CURRENT>                            0
<NET-CHANGE-FROM-OPS>                        7,092,777
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                  (4,508,136)
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                    343,914,632
<NUMBER-OF-SHARES-REDEEMED>              (312,471,091)
<SHARES-REINVESTED>                          4,316,891
<NET-CHANGE-IN-ASSETS>                      38,345,073
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                     (27,255)
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                        (162,000)
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                            (1,279,000)
<AVERAGE-NET-ASSETS>                       325,730,000
<PER-SHARE-NAV-BEGIN>                            1.000
<PER-SHARE-NII>                                  0.020
<PER-SHARE-GAIN-APPREC>                          0.000
<PER-SHARE-DIVIDEND>                           (0.020)
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                              1.000
<EXPENSE-RATIO>                                 0.5000


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 2
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                       51,484,799
<INVESTMENTS-AT-VALUE>                      50,817,037
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<TOTAL-ASSETS>                              53,038,713
<PAYABLE-FOR-SECURITIES>                     1,574,679
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                       55,936
<TOTAL-LIABILITIES>                          1,630,615
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                    57,776,416
<SHARES-COMMON-STOCK>                        5,290,804
<SHARES-COMMON-PRIOR>                        4,479,005
<ACCUMULATED-NII-CURRENT>                    1,483,444
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                   (7,184,000)
<ACCUM-APPREC-OR-DEPREC>                     (667,762)
<NET-ASSETS>                                51,408,098
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            1,647,157
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (163,713)
<NET-INVESTMENT-INCOME>                      1,483,444
<REALIZED-GAINS-CURRENT>                      (93,952)
<APPREC-INCREASE-CURRENT>                    (885,681)
<NET-CHANGE-FROM-OPS>                          503,811
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                        115,232
<NUMBER-OF-SHARES-REDEEMED>                  (177,997)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                       8,113,437
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                           41,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                223,000
<AVERAGE-NET-ASSETS>                        53,359,000
<PER-SHARE-NAV-BEGIN>                            9.430
<PER-SHARE-NII>                                  0.270
<PER-SHARE-GAIN-APPREC>                        (0.190)
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<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                              9.510
<EXPENSE-RATIO>                                 0.8200


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS, INC.
<SERIES>
   <NUMBER> 2
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                       51,484,799
<INVESTMENTS-AT-VALUE>                      50,817,037
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<PAID-IN-CAPITAL-COMMON>                    57,776,416
<SHARES-COMMON-STOCK>                          114,391
<SHARES-COMMON-PRIOR>                          135,351
<ACCUMULATED-NII-CURRENT>                    1,483,444
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                   (7,184,000)
<ACCUM-APPREC-OR-DEPREC>                     (667,762)
<NET-ASSETS>                                51,408,098
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            1,647,157
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (163,713)
<NET-INVESTMENT-INCOME>                      1,483,444
<REALIZED-GAINS-CURRENT>                      (93,952)
<APPREC-INCREASE-CURRENT>                    (885,681)
<NET-CHANGE-FROM-OPS>                          503,811
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                         30,650
<NUMBER-OF-SHARES-REDEEMED>                   (51,610)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                         305,604
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                           41,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                223,000
<AVERAGE-NET-ASSETS>                         1,175,000
<PER-SHARE-NAV-BEGIN>                            9.410
<PER-SHARE-NII>                                  0.280
<PER-SHARE-GAIN-APPREC>                        (0.210)
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<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                              9.480
<EXPENSE-RATIO>                                 1.0700


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 3
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      185,007,003
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<SENIOR-LONG-TERM-DEBT>                              0
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<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   193,096,239
<SHARES-COMMON-STOCK>                       18,422,790
<SHARES-COMMON-PRIOR>                       18,396,881
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<OVERDISTRIBUTION-NII>                               0
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<ACCUM-APPREC-OR-DEPREC>                   (4,709,498)
<NET-ASSETS>                               182,695,326
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            5,812,631
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<EXPENSES-NET>                               (702,868)
<NET-INVESTMENT-INCOME>                      5,109,763
<REALIZED-GAINS-CURRENT>                   (3,529,438)
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<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
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<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                        936,096
<NUMBER-OF-SHARES-REDEEMED>                  (910,186)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                     (4,065,324)
<ACCUMULATED-NII-PRIOR>                          1,610
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                   (149,238)
<GROSS-ADVISORY-FEES>                          186,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                796,000
<AVERAGE-NET-ASSETS>                       182,860,000
<PER-SHARE-NAV-BEGIN>                            9.910
<PER-SHARE-NII>                                  0.000
<PER-SHARE-GAIN-APPREC>                        (0.220)
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<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                              9.690
<EXPENSE-RATIO>                                 0.7500


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 3
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      185,007,003
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<SENIOR-LONG-TERM-DEBT>                              0
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<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   193,096,239
<SHARES-COMMON-STOCK>                          438,395
<SHARES-COMMON-PRIOR>                          434,072
<ACCUMULATED-NII-CURRENT>                    5,111,373
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<ACCUMULATED-NET-GAINS>                              0
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<ACCUM-APPREC-OR-DEPREC>                   (4,709,498)
<NET-ASSETS>                               182,695,326
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            5,812,631
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (702,868)
<NET-INVESTMENT-INCOME>                      5,109,763
<REALIZED-GAINS-CURRENT>                   (3,529,438)
<APPREC-INCREASE-CURRENT>                  (5,942,833)
<NET-CHANGE-FROM-OPS>                      (4,362,508)
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
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<NUMBER-OF-SHARES-SOLD>                        413,304
<NUMBER-OF-SHARES-REDEEMED>                  (408,980)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                     (4,293,648)
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<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                   (149,238)
<GROSS-ADVISORY-FEES>                          186,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                796,000
<AVERAGE-NET-ASSETS>                         4,594,000
<PER-SHARE-NAV-BEGIN>                            9.880
<PER-SHARE-NII>                                  0.000
<PER-SHARE-GAIN-APPREC>                        (0.240)
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<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                              9.640
<EXPENSE-RATIO>                                 1.0000


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 4
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      154,800,576
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<PAYABLE-FOR-SECURITIES>                     1,190,484
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      295,635
<TOTAL-LIABILITIES>                          1,486,119
<SENIOR-EQUITY>                            155,906,257
<PAID-IN-CAPITAL-COMMON>                   154,860,974
<SHARES-COMMON-STOCK>                       14,247,829
<SHARES-COMMON-PRIOR>                       14,300,122
<ACCUMULATED-NII-CURRENT>                    5,168,358
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<OVERDISTRIBUTION-GAINS>                   (3,216,266)
<ACCUM-APPREC-OR-DEPREC>                     (906,809)
<NET-ASSETS>                               155,906,257
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            5,575,674
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (407,316)
<NET-INVESTMENT-INCOME>                      5,168,358
<REALIZED-GAINS-CURRENT>                   (1,580,582)
<APPREC-INCREASE-CURRENT>                  (7,432,437)
<NET-CHANGE-FROM-OPS>                      (3,844,661)
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                      1,935,292
<NUMBER-OF-SHARES-REDEEMED>                (1,987,585)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                     (4,310,051)
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                 (1,635,684)
<GROSS-ADVISORY-FEES>                          403,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                502,000
<AVERAGE-NET-ASSETS>                       162,356,000
<PER-SHARE-NAV-BEGIN>                           10.930
<PER-SHARE-NII>                                  0.350
<PER-SHARE-GAIN-APPREC>                        (0.610)
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             10.670
<EXPENSE-RATIO>                                 0.5000


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 4
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
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<SENIOR-EQUITY>                            155,906,257
<PAID-IN-CAPITAL-COMMON>                   154,860,974
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<SHARES-COMMON-PRIOR>                          355,243
<ACCUMULATED-NII-CURRENT>                    5,168,358
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                   (3,216,266)
<ACCUM-APPREC-OR-DEPREC>                     (906,809)
<NET-ASSETS>                               155,906,257
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            5,575,674
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (407,316)
<NET-INVESTMENT-INCOME>                      5,168,358
<REALIZED-GAINS-CURRENT>                   (1,580,582)
<APPREC-INCREASE-CURRENT>                  (7,432,437)
<NET-CHANGE-FROM-OPS>                      (3,844,661)
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                         63,698
<NUMBER-OF-SHARES-REDEEMED>                   (55,064)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                     (3,753,124)
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                 (1,635,684)
<GROSS-ADVISORY-FEES>                          403,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                502,000
<AVERAGE-NET-ASSETS>                       162,356,000
<PER-SHARE-NAV-BEGIN>                            10.92
<PER-SHARE-NII>                                  0.340
<PER-SHARE-GAIN-APPREC>                        (0.610)
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             10.650
<EXPENSE-RATIO>                                 0.7500


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 5
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      489,820,452
<INVESTMENTS-AT-VALUE>                     547,645,095
<RECEIVABLES>                               28,606,611
<ASSETS-OTHER>                                 632,276
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             576,883,982
<PAYABLE-FOR-SECURITIES>                    25,472,287
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      505,565
<TOTAL-LIABILITIES>                         25,977,852
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   410,502,701
<SHARES-COMMON-STOCK>                       32,327,798
<SHARES-COMMON-PRIOR>                       32,588,515
<ACCUMULATED-NII-CURRENT>                    9,064,674
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                     72,804,017
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                    58,534,738
<NET-ASSETS>                               523,492,077
<DIVIDEND-INCOME>                            1,217,146
<INTEREST-INCOME>                            9,782,967
<OTHER-INCOME>                                       0
<EXPENSES-NET>                             (2,124,180)
<NET-INVESTMENT-INCOME>                      8,875,933
<REALIZED-GAINS-CURRENT>                    20,331,365
<APPREC-INCREASE-CURRENT>                    (591,502)
<NET-CHANGE-FROM-OPS>                       28,615,796
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                      1,907,856
<NUMBER-OF-SHARES-REDEEMED>                (2,168,573)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                      24,414,649
<ACCUMULATED-NII-PRIOR>                        188,741
<ACCUMULATED-GAINS-PRIOR>                   52,472,652
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                          402,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              2,124,000
<AVERAGE-NET-ASSETS>                       540,069,000
<PER-SHARE-NAV-BEGIN>                           15.360
<PER-SHARE-NII>                                  0.260
<PER-SHARE-GAIN-APPREC>                          0.570
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             16.190
<EXPENSE-RATIO>                                 0.7800


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTINAL FUNDS INC.
<SERIES>
   <NUMBER> 5
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      489,820,452
<INVESTMENTS-AT-VALUE>                     547,645,095
<RECEIVABLES>                               28,606,611
<ASSETS-OTHER>                                 632,276
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             576,883,982
<PAYABLE-FOR-SECURITIES>                    25,472,287
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      505,565
<TOTAL-LIABILITIES>                         25,977,852
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   410,502,701
<SHARES-COMMON-STOCK>                        1,698,334
<SHARES-COMMON-PRIOR>                        1,099,471
<ACCUMULATED-NII-CURRENT>                    9,064,674
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                     72,804,017
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                    58,534,738
<NET-ASSETS>                                27,414,053
<DIVIDEND-INCOME>                            1,217,146
<INTEREST-INCOME>                            9,782,967
<OTHER-INCOME>                                       0
<EXPENSES-NET>                             (2,124,180)
<NET-INVESTMENT-INCOME>                      8,875,933
<REALIZED-GAINS-CURRENT>                    20,331,365
<APPREC-INCREASE-CURRENT>                    (591,502)
<NET-CHANGE-FROM-OPS>                       28,615,796
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                        716,137
<NUMBER-OF-SHARES-REDEEMED>                  (117,274)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                      37,805,484
<ACCUMULATED-NII-PRIOR>                        188,741
<ACCUMULATED-GAINS-PRIOR>                   52,472,652
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                          402,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              2,124,000
<AVERAGE-NET-ASSETS>                       540,069,000
<PER-SHARE-NAV-BEGIN>                           15.330
<PER-SHARE-NII>                                  0.260
<PER-SHARE-GAIN-APPREC>                          0.550
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             16.140
<EXPENSE-RATIO>                                 1.0300


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> THE MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 6
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      725,380,450
<INVESTMENTS-AT-VALUE>                     816,439,277
<RECEIVABLES>                               20,394,130
<ASSETS-OTHER>                                   3,914
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             836,837,321
<PAYABLE-FOR-SECURITIES>                    17,017,856
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      938,585
<TOTAL-LIABILITIES>                         17,956,441
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   752,623,566
<SHARES-COMMON-STOCK>                          697,141
<SHARES-COMMON-PRIOR>                          828,184
<ACCUMULATED-NII-CURRENT>                    3,827,584
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                   (28,694,822)
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                    91,124,552
<NET-ASSETS>                               818,880,880
<DIVIDEND-INCOME>                            7,125,951
<INTEREST-INCOME>                              519,544
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               3,825,067
<NET-INVESTMENT-INCOME>                      3,820,428
<REALIZED-GAINS-CURRENT>                  (20,106,258)
<APPREC-INCREASE-CURRENT>                  126,330,177
<NET-CHANGE-FROM-OPS>                      110,044,347
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                         46,844
<NUMBER-OF-SHARES-REDEEMED>                  (177,887)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                       (301,382)
<ACCUMULATED-NII-PRIOR>                          7,156
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                 (8,588,564)
<GROSS-ADVISORY-FEES>                          986,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              3,825,000
<AVERAGE-NET-ASSETS>                       794,897,000
<PER-SHARE-NAV-BEGIN>                           11.760
<PER-SHARE-NII>                                  0.030
<PER-SHARE-GAIN-APPREC>                          1.750
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             13.540
<EXPENSE-RATIO>                                  1.220


</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> THE MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 6
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      725,380,450
<INVESTMENTS-AT-VALUE>                     816,439,127
<RECEIVABLES>                               20,394,130
<ASSETS-OTHER>                                   3,914
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             836,837,321
<PAYABLE-FOR-SECURITIES>                    17,017,856
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      938,585
<TOTAL-LIABILITIES>                         17,956,441
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   752,623,566
<SHARES-COMMON-STOCK>                       59,701,702
<SHARES-COMMON-PRIOR>                       68,093,903
<ACCUMULATED-NII-CURRENT>                    3,827,584
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                   (28,694,822)
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                    91,124,552
<NET-ASSETS>                               818,880,880
<DIVIDEND-INCOME>                            7,125,951
<INTEREST-INCOME>                              519,544
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               3,825,067
<NET-INVESTMENT-INCOME>                      3,820,428
<REALIZED-GAINS-CURRENT>                  (20,106,258)
<APPREC-INCREASE-CURRENT>                  126,330,177
<NET-CHANGE-FROM-OPS>                      110,044,347
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                      2,873,392
<NUMBER-OF-SHARES-REDEEMED>               (11,265,593)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                       8,449,585
<ACCUMULATED-NII-PRIOR>                          7,156
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                 (8,588,564)
<GROSS-ADVISORY-FEES>                          986,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              3,825,000
<AVERAGE-NET-ASSETS>                       794,897,000
<PER-SHARE-NAV-BEGIN>                           11.760
<PER-SHARE-NII>                                  0.060
<PER-SHARE-GAIN-APPREC>                          1.740
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                              0.00
<PER-SHARE-NAV-END>                             13.560
<EXPENSE-RATIO>                                  0.970


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> THE MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 7
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      516,557,810
<INVESTMENTS-AT-VALUE>                   1,059,641,655
<RECEIVABLES>                                3,112,862
<ASSETS-OTHER>                                   1,117
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                           1,062,755,634
<PAYABLE-FOR-SECURITIES>                     5,002,363
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                    1,905,521
<TOTAL-LIABILITIES>                          6,907,884
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   444,873,160
<SHARES-COMMON-STOCK>                          471,946
<SHARES-COMMON-PRIOR>                          477,407
<ACCUMULATED-NII-CURRENT>                  (2,353,985)
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                     70,244,730
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                   543,083,845
<NET-ASSETS>                             1,055,847,750
<DIVIDEND-INCOME>                            2,012,118
<INTEREST-INCOME>                              407,322
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               4,773,425
<NET-INVESTMENT-INCOME>                    (2,353,985)
<REALIZED-GAINS-CURRENT>                    60,358,259
<APPREC-INCREASE-CURRENT>                   17,256,393
<NET-CHANGE-FROM-OPS>                       75,260,667
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                         62,661
<NUMBER-OF-SHARES-REDEEMED>                   (68,122)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                         989,106
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                    9,886,471
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                        1,274,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              4,773,000
<AVERAGE-NET-ASSETS>                     1,027,161,000
<PER-SHARE-NAV-BEGIN>                           33.130
<PER-SHARE-NII>                                (0.120)
<PER-SHARE-GAIN-APPREC>                          2.590
<PER-SHARE-DIVIDEND>                              0.00
<PER-SHARE-DISTRIBUTIONS>                         0.00
<RETURNS-OF-CAPITAL>                              0.00
<PER-SHARE-NAV-END>                             35.600
<EXPENSE-RATIO>                                  1.180


</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> THE MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 7
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      516,557,810
<INVESTMENTS-AT-VALUE>                   1,059,641,655
<RECEIVABLES>                                3,112,862
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<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                           1,062,755,634
<PAYABLE-FOR-SECURITIES>                     5,002,363
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                    1,905,521
<TOTAL-LIABILITIES>                          6,907,884
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   444,873,160
<SHARES-COMMON-STOCK>                       28,835,947
<SHARES-COMMON-PRIOR>                       29,121,360
<ACCUMULATED-NII-CURRENT>                  (2,353,985)
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                     70,244,730
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                   543,083,845
<NET-ASSETS>                             1,055,847,750
<DIVIDEND-INCOME>                            2,012,118
<INTEREST-INCOME>                              407,322
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               4,773,425
<NET-INVESTMENT-INCOME>                    (2,353,985)
<REALIZED-GAINS-CURRENT>                    60,358,259
<APPREC-INCREASE-CURRENT>                   17,256,393
<NET-CHANGE-FROM-OPS>                       75,260,667
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
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<NUMBER-OF-SHARES-SOLD>                      4,683,217
<NUMBER-OF-SHARES-REDEEMED>                (4,968,630)
<SHARES-REINVESTED>                                  0
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<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
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<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              4,773,000
<AVERAGE-NET-ASSETS>                     1,027,161,000
<PER-SHARE-NAV-BEGIN>                           33.480
<PER-SHARE-NII>                                (0.080)
<PER-SHARE-GAIN-APPREC>                          2.630
<PER-SHARE-DIVIDEND>                              0.00
<PER-SHARE-DISTRIBUTIONS>                         0.00
<RETURNS-OF-CAPITAL>                              0.00
<PER-SHARE-NAV-END>                             36.030
<EXPENSE-RATIO>                                  0.930


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 8
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      909,560,754
<INVESTMENTS-AT-VALUE>                   1,648,973,105
<RECEIVABLES>                               36,593,725
<ASSETS-OTHER>                                       0
<OTHER-ITEMS-ASSETS>                           868,419
<TOTAL-ASSETS>                           1,686,435,249
<PAYABLE-FOR-SECURITIES>                    21,106,711
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                    7,032,657
<TOTAL-LIABILITIES>                         28,139,368
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   834,765,433
<SHARES-COMMON-STOCK>                       42,868,861
<SHARES-COMMON-PRIOR>                       44,093,115
<ACCUMULATED-NII-CURRENT>                    8,618,695
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                     74,606,752
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                   740,305,001
<NET-ASSETS>                             1,605,504,517
<DIVIDEND-INCOME>                           10,508,125
<INTEREST-INCOME>                              515,995
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<EXPENSES-NET>                             (2,405,425)
<NET-INVESTMENT-INCOME>                      8,618,695
<REALIZED-GAINS-CURRENT>                    61,411,154
<APPREC-INCREASE-CURRENT>                  109,632,516
<NET-CHANGE-FROM-OPS>                      179,662,365
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                      3,962,571
<NUMBER-OF-SHARES-REDEEMED>                (5,186,825)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                     138,018,597
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                   13,195,598
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                          784,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              4,354,000
<AVERAGE-NET-ASSETS>                     1,581,633,000
<PER-SHARE-NAV-BEGIN>                           33.390
<PER-SHARE-NII>                                  0.190
<PER-SHARE-GAIN-APPREC>                          3.870
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             37.450
<EXPENSE-RATIO>                                  0.300


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 8
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      909,560,754
<INVESTMENTS-AT-VALUE>                   1,648,973,105
<RECEIVABLES>                               36,593,725
<ASSETS-OTHER>                                       0
<OTHER-ITEMS-ASSETS>                           868,419
<TOTAL-ASSETS>                           1,686,435,249
<PAYABLE-FOR-SECURITIES>                             0
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                    7,032,657
<TOTAL-LIABILITIES>                          7,032,657
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   834,765,433
<SHARES-COMMON-STOCK>                        1,415,878
<SHARES-COMMON-PRIOR>                        1,094,877
<ACCUMULATED-NII-CURRENT>                    8,618,695
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                     74,606,752
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                   740,305,001
<NET-ASSETS>                                52,791,364
<DIVIDEND-INCOME>                           10,508,125
<INTEREST-INCOME>                              515,995
<OTHER-INCOME>                                       0
<EXPENSES-NET>                             (2,405,425)
<NET-INVESTMENT-INCOME>                      8,618,695
<REALIZED-GAINS-CURRENT>                    61,411,154
<APPREC-INCREASE-CURRENT>                  109,632,516
<NET-CHANGE-FROM-OPS>                      179,662,365
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                        503,211
<NUMBER-OF-SHARES-REDEEMED>                  (182,210)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                     191,234,276
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                   13,195,598
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                          784,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              4,354,000
<AVERAGE-NET-ASSETS>                     1,581,633,000
<PER-SHARE-NAV-BEGIN>                           33.280
<PER-SHARE-NII>                                  0.190
<PER-SHARE-GAIN-APPREC>                          3.820
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             37.290
<EXPENSE-RATIO>                                 0.5500


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 9
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                       62,480,796
<INVESTMENTS-AT-VALUE>                      78,435,944
<RECEIVABLES>                                7,417,394
<ASSETS-OTHER>                                 434,787
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                              86,288,025
<PAYABLE-FOR-SECURITIES>                    10,123,733
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      165,031
<TOTAL-LIABILITIES>                         10,288,764
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                    58,868,323
<SHARES-COMMON-STOCK>                        6,388,551
<SHARES-COMMON-PRIOR>                        5,776,575
<ACCUMULATED-NII-CURRENT>                            0
<OVERDISTRIBUTION-NII>                       (276,124)
<ACCUMULATED-NET-GAINS>                      1,438,745
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                    15,968,317
<NET-ASSETS>                                75,999,261
<DIVIDEND-INCOME>                              865,389
<INTEREST-INCOME>                               47,272
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (321,336)
<NET-INVESTMENT-INCOME>                        591,325
<REALIZED-GAINS-CURRENT>                     1,152,148
<APPREC-INCREASE-CURRENT>                    1,515,837
<NET-CHANGE-FROM-OPS>                        3,259,310
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                      3,759,674
<NUMBER-OF-SHARES-REDEEMED>                (3,147,698)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                      10,391,220
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                      286,597
<OVERDISTRIB-NII-PRIOR>                      (867,449)
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                           51,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                439,000
<AVERAGE-NET-ASSETS>                        68,795,000
<PER-SHARE-NAV-BEGIN>                           11.270
<PER-SHARE-NII>                                  0.110
<PER-SHARE-GAIN-APPREC>                          0.430
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             11.810
<EXPENSE-RATIO>                                  0.940


</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUND INC.
<SERIES>
   <NUMBER> 9
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                       62,480,796
<INVESTMENTS-AT-VALUE>                      78,435,944
<RECEIVABLES>                                7,417,394
<ASSETS-OTHER>                                 434,787
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                              86,288,025
<PAYABLE-FOR-SECURITIES>                    10,123,733
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      165,031
<TOTAL-LIABILITIES>                         10,288,764
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                    58,868,323
<SHARES-COMMON-STOCK>                           46,477
<SHARES-COMMON-PRIOR>                           44,486
<ACCUMULATED-NII-CURRENT>                            0
<OVERDISTRIBUTION-NII>                       (276,124)
<ACCUMULATED-NET-GAINS>                      1,438,745
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                    15,968,317
<NET-ASSETS>                                75,999,261
<DIVIDEND-INCOME>                              865,389
<INTEREST-INCOME>                               47,272
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (321,336)
<NET-INVESTMENT-INCOME>                        591,325
<REALIZED-GAINS-CURRENT>                     1,152,148
<APPREC-INCREASE-CURRENT>                    1,515,837
<NET-CHANGE-FROM-OPS>                        3,259,310
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                          5,251
<NUMBER-OF-SHARES-REDEEMED>                    (3,259)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                       3,281,422
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                      286,597
<OVERDISTRIB-NII-PRIOR>                      (867,449)
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                           51,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                439,000
<AVERAGE-NET-ASSETS>                        68,795,000
<PER-SHARE-NAV-BEGIN>                           11.220
<PER-SHARE-NII>                                  0.090
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<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             11.750
<EXPENSE-RATIO>                                  1.190


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 10
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                       61,088,836
<INVESTMENTS-AT-VALUE>                      58,056,936
<RECEIVABLES>                                9,561,072
<ASSETS-OTHER>                               2,570,073
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                              70,188,081
<PAYABLE-FOR-SECURITIES>                    11,562,337
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      499,282
<TOTAL-LIABILITIES>                         12,061,619
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                    61,783,075
<SHARES-COMMON-STOCK>                        6,036,681
<SHARES-COMMON-PRIOR>                        5,924,807
<ACCUMULATED-NII-CURRENT>                    1,488,204
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                   (1,963,582)
<ACCUM-APPREC-OR-DEPREC>                   (3,181,235)
<NET-ASSETS>                                58,126,462
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            1,533,342
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (280,360)
<NET-INVESTMENT-INCOME>                      1,252,982
<REALIZED-GAINS-CURRENT>                   (2,738,546)
<APPREC-INCREASE-CURRENT>                  (4,149,118)
<NET-CHANGE-FROM-OPS>                      (5,634,682)
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                        305,803
<NUMBER-OF-SHARES-REDEEMED>                  (193,929)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                     (4,649,950)
<ACCUMULATED-NII-PRIOR>                        235,222
<ACCUMULATED-GAINS-PRIOR>                      774,964
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                           88,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                304,000
<AVERAGE-NET-ASSETS>                        59,454,000
<PER-SHARE-NAV-BEGIN>                           10.560
<PER-SHARE-NII>                                  0.210
<PER-SHARE-GAIN-APPREC>                        (1.180)
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                              9.590
<EXPENSE-RATIO>                                 0.9500


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 10
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                       61,088,836
<INVESTMENTS-AT-VALUE>                      58,056,936
<RECEIVABLES>                                9,561,072
<ASSETS-OTHER>                               2,570,073
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                              70,188,081
<PAYABLE-FOR-SECURITIES>                    11,562,337
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      499,282
<TOTAL-LIABILITIES>                         12,061,619
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                    61,783,075
<SHARES-COMMON-STOCK>                           21,622
<SHARES-COMMON-PRIOR>                           22,241
<ACCUMULATED-NII-CURRENT>                    1,488,204
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                   (1,963,582)
<ACCUM-APPREC-OR-DEPREC>                   (3,181,235)
<NET-ASSETS>                                62,782,603
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            1,533,342
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (280,360)
<NET-INVESTMENT-INCOME>                      1,252,982
<REALIZED-GAINS-CURRENT>                   (2,738,546)
<APPREC-INCREASE-CURRENT>                  (4,149,118)
<NET-CHANGE-FROM-OPS>                      (5,634,682)
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                             82
<NUMBER-OF-SHARES-REDEEMED>                      (700)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                     (5,640,873)
<ACCUMULATED-NII-PRIOR>                        235,222
<ACCUMULATED-GAINS-PRIOR>                      774,964
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                           88,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                304,000
<AVERAGE-NET-ASSETS>                        59,454,000
<PER-SHARE-NAV-BEGIN>                           10.520
<PER-SHARE-NII>                                  0.210
<PER-SHARE-GAIN-APPREC>                        (1.180)
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                              9.550
<EXPENSE-RATIO>                                 1.2000


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 11
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      113,573,101
<INVESTMENTS-AT-VALUE>                     136,872,712
<RECEIVABLES>                                3,283,443
<ASSETS-OTHER>                               3,592,102
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             143,748,257
<PAYABLE-FOR-SECURITIES>                     2,908,857
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      844,463
<TOTAL-LIABILITIES>                          3,753,320
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   113,373,347
<SHARES-COMMON-STOCK>                       11,536,086
<SHARES-COMMON-PRIOR>                       11,902,743
<ACCUMULATED-NII-CURRENT>                            0
<OVERDISTRIBUTION-NII>                       (373,071)
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                     3,561,447
<ACCUM-APPREC-OR-DEPREC>                    23,433,214
<NET-ASSETS>                               139,994,937
<DIVIDEND-INCOME>                            1,782,440
<INTEREST-INCOME>                               35,010
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (693,723)
<NET-INVESTMENT-INCOME>                      1,123,727
<REALIZED-GAINS-CURRENT>                     6,984,150
<APPREC-INCREASE-CURRENT>                  (4,811,164)
<NET-CHANGE-FROM-OPS>                        3,296,713
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                        974,617
<NUMBER-OF-SHARES-REDEEMED>                (1,341,274)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                     (1,243,887)
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                    (1,496,798)
<OVERDIST-NET-GAINS-PRIOR>                 (3,422,703)
<GROSS-ADVISORY-FEES>                          244,987
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                694,000
<AVERAGE-NET-ASSETS>                       141,152,000
<PER-SHARE-NAV-BEGIN>                           11.810
<PER-SHARE-NII>                                  0.090
<PER-SHARE-GAIN-APPREC>                          0.180
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             12.080
<EXPENSE-RATIO>                                 0.9900


</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK>  0000868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 11
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<INVESTMENTS-AT-COST>                      113,573,101
<INVESTMENTS-AT-VALUE>                     136,872,712
<RECEIVABLES>                                3,283,443
<ASSETS-OTHER>                               3,592,102
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             143,748,257
<PAYABLE-FOR-SECURITIES>                     2,908,857
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      844,463
<TOTAL-LIABILITIES>                          3,753,320
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   113,373,347
<SHARES-COMMON-STOCK>                           52,148
<SHARES-COMMON-PRIOR>                           57,595
<ACCUMULATED-NII-CURRENT>                            0
<OVERDISTRIBUTION-NII>                       (373,071)
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                     3,561,447
<ACCUM-APPREC-OR-DEPREC>                    23,433,214
<NET-ASSETS>                               139,994,937
<DIVIDEND-INCOME>                            1,782,440
<INTEREST-INCOME>                               35,010
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (693,723)
<NET-INVESTMENT-INCOME>                      1,123,727
<REALIZED-GAINS-CURRENT>                     6,984,150
<APPREC-INCREASE-CURRENT>                  (4,811,164)
<NET-CHANGE-FROM-OPS>                        3,296,713
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                          8,129
<NUMBER-OF-SHARES-REDEEMED>                   (13,576)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                       3,230,506
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                    (1,496,798)
<OVERDIST-NET-GAINS-PRIOR>                 (3,422,703)
<GROSS-ADVISORY-FEES>                          244,987
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                694,000
<AVERAGE-NET-ASSETS>                       141,152,000
<PER-SHARE-NAV-BEGIN>                           11.720
<PER-SHARE-NII>                                  0.090
<PER-SHARE-GAIN-APPREC>                          0.160
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             11.970
<EXPENSE-RATIO>                                 1.2400


</TABLE>


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