MAINSTAY INSTITUTIONAL FUNDS INC
NSAR-B, 1999-12-29
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<PAGE>      PAGE  1
000 B000000 10/31/99
000 C000000 868307
000 D000000 N
000 E000000 F
000 F000000 Y
000 G000000 N
000 H000000 N
000 I000000 3.0.a
000 J000000 U
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
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008 B000101 S
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<PAGE>      PAGE  4
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SIGNATURE   CYNTHIA BAUGHMAN
TITLE       ATTORNEY


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

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 1
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

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

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 1
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
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</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 1
   <NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   10-MOS
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</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTION FUNDS INC.
<SERIES>
   <NUMBER> 5
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
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</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 5
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   10-MOS
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<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                      482,248,437
<INVESTMENTS-AT-VALUE>                     539,012,033
<RECEIVABLES>                                2,434,995
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<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             543,869,634
<PAYABLE-FOR-SECURITIES>                       292,064
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      630,846
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<SENIOR-EQUITY>                                      0
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<SHARES-COMMON-PRIOR>                        1,099,471
<ACCUMULATED-NII-CURRENT>                   14,697,321
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                     16,140,333
<OVERDISTRIBUTION-GAINS>                             0
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<NET-ASSETS>                                29,086,881
<DIVIDEND-INCOME>                            1,869,673
<INTEREST-INCOME>                           17,506,135
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<EXPENSES-NET>                             (4,138,882)
<NET-INVESTMENT-INCOME>                     15,236,926
<REALIZED-GAINS-CURRENT>                    15,712,596
<APPREC-INCREASE-CURRENT>                  (1,884,217)
<NET-CHANGE-FROM-OPS>                       29,065,305
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                      (5,609)
<DISTRIBUTIONS-OF-GAINS>                   (2,839,920)
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                      1,008,664
<NUMBER-OF-SHARES-REDEEMED>                  (307,680)
<SHARES-REINVESTED>                            205,157
<NET-CHANGE-IN-ASSETS>                      39,890,081
<ACCUMULATED-NII-PRIOR>                        188,741
<ACCUMULATED-GAINS-PRIOR>                   52,472,652
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                          676,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              4,139,000
<AVERAGE-NET-ASSETS>                       540,784,000
<PER-SHARE-NAV-BEGIN>                           15.330
<PER-SHARE-NII>                                  0.400
<PER-SHARE-GAIN-APPREC>                          0.340
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                      (1.570)
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             14.500
<EXPENSE-RATIO>                                  1.030


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

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> THE MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 6
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   10-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                      690,330,738
<INVESTMENTS-AT-VALUE>                     727,830,010
<RECEIVABLES>                               21,867,556
<ASSETS-OTHER>                                   3,475
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             749,701,041
<PAYABLE-FOR-SECURITIES>                             0
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      983,638
<TOTAL-LIABILITIES>                            983,638
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   733,908,249
<SHARES-COMMON-STOCK>                       58,409,114
<SHARES-COMMON-PRIOR>                       68,093,903
<ACCUMULATED-NII-CURRENT>                    5,050,776
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                   (27,603,548)
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                    37,361,926
<NET-ASSETS>                               748,717,403
<DIVIDEND-INCOME>                           10,772,059
<INTEREST-INCOME>                            1,064,057
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               6,319,593
<NET-INVESTMENT-INCOME>                      5,516,523
<REALIZED-GAINS-CURRENT>                  (19,476,120)
<APPREC-INCREASE-CURRENT>                   72,567,551
<NET-CHANGE-FROM-OPS>                       58,607,954
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                     (11,708)
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                      3,552,326
<NUMBER-OF-SHARES-REDEEMED>               (13,241,066)
<SHARES-REINVESTED>                              3,951
<NET-CHANGE-IN-ASSETS>                    (62,015,274)
<ACCUMULATED-NII-PRIOR>                          7,156
<ACCUMULATED-GAINS-PRIOR>                  (8,588,564)
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                        1,643,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              6,320,000
<AVERAGE-NET-ASSETS>                       789,033,000
<PER-SHARE-NAV-BEGIN>                           11.760
<PER-SHARE-NII>                                  0.090
<PER-SHARE-GAIN-APPREC>                          0.840
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                              0.00
<PER-SHARE-NAV-END>                             12.690
<EXPENSE-RATIO>                                  0.960


</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> THE MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 6
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   10-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                      690,330,738
<INVESTMENTS-AT-VALUE>                     727,830,010
<RECEIVABLES>                               21,867,556
<ASSETS-OTHER>                                   3,475
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             749,701,041
<PAYABLE-FOR-SECURITIES>                             0
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      983,638
<TOTAL-LIABILITIES>                            983,638
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   733,908,249
<SHARES-COMMON-STOCK>                          585,816
<SHARES-COMMON-PRIOR>                          828,184
<ACCUMULATED-NII-CURRENT>                    5,050,776
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                   (27,603,548)
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                    37,361,926
<NET-ASSETS>                               748,717,403
<DIVIDEND-INCOME>                           10,772,059
<INTEREST-INCOME>                            1,064,057
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               6,319,593
<NET-INVESTMENT-INCOME>                      5,516,523
<REALIZED-GAINS-CURRENT>                  (19,476,120)
<APPREC-INCREASE-CURRENT>                   72,567,551
<NET-CHANGE-FROM-OPS>                       58,607,954
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                         (59)
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                         72,518
<NUMBER-OF-SHARES-REDEEMED>                  (314,891)
<SHARES-REINVESTED>                                  5
<NET-CHANGE-IN-ASSETS>                    (62,015,274)
<ACCUMULATED-NII-PRIOR>                          7,156
<ACCUMULATED-GAINS-PRIOR>                  (8,588,564)
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                        1,643,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              6,320,000
<AVERAGE-NET-ASSETS>                       789,033,000
<PER-SHARE-NAV-BEGIN>                           11.760
<PER-SHARE-NII>                                  0.050
<PER-SHARE-GAIN-APPREC>                          0.850
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                              0.00
<PER-SHARE-NAV-END>                             12.660
<EXPENSE-RATIO>                                  1.210


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

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> THE MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 7
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   10-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                      545,355,216
<INVESTMENTS-AT-VALUE>                   1,073,438,933
<RECEIVABLES>                                9,046,796
<ASSETS-OTHER>                                   2,245
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                           1,082,487,974
<PAYABLE-FOR-SECURITIES>                     3,670,143
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                    3,075,072
<TOTAL-LIABILITIES>                          6,745,215
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   447,481,363
<SHARES-COMMON-STOCK>                          722,100
<SHARES-COMMON-PRIOR>                          477,407
<ACCUMULATED-NII-CURRENT>                            0
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                    100,177,679
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                   528,083,717
<NET-ASSETS>                             1,075,742,759
<DIVIDEND-INCOME>                            3,138,017
<INTEREST-INCOME>                              661,429
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               8,078,868
<NET-INVESTMENT-INCOME>                    (4,279,422)
<REALIZED-GAINS-CURRENT>                   100,178,405
<APPREC-INCREASE-CURRENT>                    2,256,265
<NET-CHANGE-FROM-OPS>                       98,155,248
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                       241,486
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                        365,037
<NUMBER-OF-SHARES-REDEEMED>                  (127,190)
<SHARES-REINVESTED>                              6,846
<NET-CHANGE-IN-ASSETS>                      84,918,284
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                    9,886,471
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                        2,151,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              8,078,000
<AVERAGE-NET-ASSETS>                         1,033,000
<PER-SHARE-NAV-BEGIN>                           33.130
<PER-SHARE-NII>                                (0.210)
<PER-SHARE-GAIN-APPREC>                          3.410
<PER-SHARE-DIVIDEND>                              0.00
<PER-SHARE-DISTRIBUTIONS>                       (0.34)
<RETURNS-OF-CAPITAL>                              0.00
<PER-SHARE-NAV-END>                             35.990
<EXPENSE-RATIO>                                  1.180


</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 7
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   10-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                      545,355,216
<INVESTMENTS-AT-VALUE>                   1,073,438,933
<RECEIVABLES>                                9,046,796
<ASSETS-OTHER>                                   2,245
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                           1,082,487,974
<PAYABLE-FOR-SECURITIES>                     3,670,143
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                    3,075,072
<TOTAL-LIABILITIES>                          6,745,215
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   447,481,363
<SHARES-COMMON-STOCK>                       28,796,030
<SHARES-COMMON-PRIOR>                       29,121,360
<ACCUMULATED-NII-CURRENT>                            0
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                    100,177,679
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                   528,083,717
<NET-ASSETS>                             1,075,742,759
<DIVIDEND-INCOME>                            3,138,017
<INTEREST-INCOME>                              661,429
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               8,078,868
<NET-INVESTMENT-INCOME>                    (4,279,422)
<REALIZED-GAINS-CURRENT>                   100,178,405
<APPREC-INCREASE-CURRENT>                    2,256,265
<NET-CHANGE-FROM-OPS>                       98,155,248
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                     9,645,711
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                      5,369,068
<NUMBER-OF-SHARES-REDEEMED>                  5,981,549
<SHARES-REINVESTED>                            287,151
<NET-CHANGE-IN-ASSETS>                      84,918,284
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<ACCUMULATED-GAINS-PRIOR>                    9,886,471
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                        2,151,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              8,078,000
<AVERAGE-NET-ASSETS>                         1,033,000
<PER-SHARE-NAV-BEGIN>                           33.480
<PER-SHARE-NII>                                (0.140)
<PER-SHARE-GAIN-APPREC>                          3.450
<PER-SHARE-DIVIDEND>                              0.00
<PER-SHARE-DISTRIBUTIONS>                       (0.34)
<RETURNS-OF-CAPITAL>                              0.00
<PER-SHARE-NAV-END>                             36.450
<EXPENSE-RATIO>                                  0.930


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

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 8
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   10-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                      978,396,557
<INVESTMENTS-AT-VALUE>                   1,703,623,469
<RECEIVABLES>                                4,698,458
<ASSETS-OTHER>                                 127,800
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                           1,708,449,727
<PAYABLE-FOR-SECURITIES>                     2,897,293
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                    2,314,697
<TOTAL-LIABILITIES>                          5,211,990
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   898,682,313
<SHARES-COMMON-STOCK>                       44,379,506
<SHARES-COMMON-PRIOR>                       44,093,115
<ACCUMULATED-NII-CURRENT>                   14,052,668
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                     64,972,936
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                   725,529,820
<NET-ASSETS>                             1,641,590,851
<DIVIDEND-INCOME>                           16,986,272
<INTEREST-INCOME>                            1,183,939
<OTHER-INCOME>                                       0
<EXPENSES-NET>                             (4,117,543)
<NET-INVESTMENT-INCOME>                     14,052,668
<REALIZED-GAINS-CURRENT>                    66,096,265
<APPREC-INCREASE-CURRENT>                   94,857,335
<NET-CHANGE-FROM-OPS>                      175,006,268
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                    13,798,796
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                      6,912,449
<NUMBER-OF-SHARES-REDEEMED>                  7,029,234
<SHARES-REINVESTED>                            403,176
<NET-CHANGE-IN-ASSETS>                     174,070,810
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                   13,347,893
<OVERDISTRIB-NII-PRIOR>                      (152,295)
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                          669,578
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              4,379,561
<AVERAGE-NET-ASSETS>                     1,607,869,150
<PER-SHARE-NAV-BEGIN>                           33.390
<PER-SHARE-NII>                                  0.320
<PER-SHARE-GAIN-APPREC>                          3.590
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                      (0.310)
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             36.990
<EXPENSE-RATIO>                                  0.300


</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 8
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   10-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                      978,396,557
<INVESTMENTS-AT-VALUE>                   1,703,623,469
<RECEIVABLES>                                4,698,458
<ASSETS-OTHER>                                 127,800
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                           1,708,449,727
<PAYABLE-FOR-SECURITIES>                     2,897,293
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                    2,314,697
<TOTAL-LIABILITIES>                          5,211,990
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   898,682,313
<SHARES-COMMON-STOCK>                        1,675,161
<SHARES-COMMON-PRIOR>                        1,094,877
<ACCUMULATED-NII-CURRENT>                   14,052,668
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                     64,972,936
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                   725,529,820
<NET-ASSETS>                                61,646,886
<DIVIDEND-INCOME>                           16,986,272
<INTEREST-INCOME>                            1,183,939
<OTHER-INCOME>                                       0
<EXPENSES-NET>                             (4,117,543)
<NET-INVESTMENT-INCOME>                     14,052,668
<REALIZED-GAINS-CURRENT>                    66,096,265
<APPREC-INCREASE-CURRENT>                   94,857,335
<NET-CHANGE-FROM-OPS>                      175,006,268
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                       520,131
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                        902,104
<NUMBER-OF-SHARES-REDEEMED>                    337,095
<SHARES-REINVESTED>                             15,275
<NET-CHANGE-IN-ASSETS>                     130,461,392
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                   13,347,893
<OVERDISTRIB-NII-PRIOR>                      (152,295)
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                          669,578
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              4,379,561
<AVERAGE-NET-ASSETS>                     1,607,869,150
<PER-SHARE-NAV-BEGIN>                           33.280
<PER-SHARE-NII>                                  0.230
<PER-SHARE-GAIN-APPREC>                          3.600
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                      (0.310)
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             36.800
<EXPENSE-RATIO>                                  0.550


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

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 2
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                    6-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                       49,326,241
<INVESTMENTS-AT-VALUE>                      48,713,878
<RECEIVABLES>                                2,866,573
<ASSETS-OTHER>                                  34,250
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                              51,614,701
<PAYABLE-FOR-SECURITIES>                     2,216,760
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      211,643
<TOTAL-LIABILITIES>                          2,428,403
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                    54,966,768
<SHARES-COMMON-STOCK>                        5,001,171
<SHARES-COMMON-PRIOR>                        4,479,005
<ACCUMULATED-NII-CURRENT>                    2,382,682
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                   (7,522,898)
<ACCUM-APPREC-OR-DEPREC>                     (612,363)
<NET-ASSETS>                                49,214,189
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            2,657,222
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (267,271)
<NET-INVESTMENT-INCOME>                      2,389,951
<REALIZED-GAINS-CURRENT>                     (440,119)
<APPREC-INCREASE-CURRENT>                    (830,282)
<NET-CHANGE-FROM-OPS>                        1,119,550
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                      2,967,826
<NUMBER-OF-SHARES-REDEEMED>                (2,445,660)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                       5,930,476
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                           66,230
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                377,000
<AVERAGE-NET-ASSETS>                        51,883,000
<PER-SHARE-NAV-BEGIN>                            9.430
<PER-SHARE-NII>                                  0.470
<PER-SHARE-GAIN-APPREC>                        (0.270)
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                              9.630
<EXPENSE-RATIO>                                  0.600


</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 2
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                    6-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                       49,326,241
<INVESTMENTS-AT-VALUE>                      48,713,878
<RECEIVABLES>                                2,866,573
<ASSETS-OTHER>                                  34,250
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                              51,614,701
<PAYABLE-FOR-SECURITIES>                     2,216,760
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      211,643
<TOTAL-LIABILITIES>                          2,428,403
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                    54,966,768
<SHARES-COMMON-STOCK>                          108,061
<SHARES-COMMON-PRIOR>                           13,351
<ACCUMULATED-NII-CURRENT>                    2,382,682
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                   (7,522,898)
<ACCUM-APPREC-OR-DEPREC>                     (612,363)
<NET-ASSETS>                                49,214,189
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            2,657,222
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (267,271)
<NET-INVESTMENT-INCOME>                      2,389,951
<REALIZED-GAINS-CURRENT>                     (440,119)
<APPREC-INCREASE-CURRENT>                    (830,282)
<NET-CHANGE-FROM-OPS>                        1,119,550
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                         31,711
<NUMBER-OF-SHARES-REDEEMED>                   (59,002)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                       (237,046)
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                           66,230
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                377,000
<AVERAGE-NET-ASSETS>                         1,130,000
<PER-SHARE-NAV-BEGIN>                            9.410
<PER-SHARE-NII>                                  0.450
<PER-SHARE-GAIN-APPREC>                        (0.270)
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                              9.590
<EXPENSE-RATIO>                                  0.850


</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 3
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                    6-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                      192,088,899
<INVESTMENTS-AT-VALUE>                     189,835,000
<RECEIVABLES>                               11,178,150
<ASSETS-OTHER>                                   7,069
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             201,020,219
<PAYABLE-FOR-SECURITIES>                    22,568,563
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      188,655
<TOTAL-LIABILITIES>                         22,757,218
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   187,598,626
<SHARES-COMMON-STOCK>                       17,908,432
<SHARES-COMMON-PRIOR>                       18,396,881
<ACCUMULATED-NII-CURRENT>                    8,597,008
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                  (15,697,028)
<ACCUM-APPREC-OR-DEPREC>                   (2,253,899)
<NET-ASSETS>                               178,244,707
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            9,709,098
<OTHER-INCOME>                                       0
<EXPENSES-NET>                             (1,165,756)
<NET-INVESTMENT-INCOME>                      8,543,342
<REALIZED-GAINS-CURRENT>                   (8,369,780)
<APPREC-INCREASE-CURRENT>                  (3,487,234)
<NET-CHANGE-FROM-OPS>                      (3,313,672)
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                      1,238,519
<NUMBER-OF-SHARES-REDEEMED>                (1,727,153)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                     (7,881,467)
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                          308,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              1,324,905
<AVERAGE-NET-ASSETS>                       108,831,606
<PER-SHARE-NAV-BEGIN>                            9.910
<PER-SHARE-NII>                                  0.470
<PER-SHARE-GAIN-APPREC>                        (0.630)
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                              9.750
<EXPENSE-RATIO>                                  0.750


</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<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>                      192,088,899
<INVESTMENTS-AT-VALUE>                     189,835,000
<RECEIVABLES>                               11,178,150
<ASSETS-OTHER>                                   7,069
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             201,020,219
<PAYABLE-FOR-SECURITIES>                    22,568,563
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      188,655
<TOTAL-LIABILITIES>                         22,757,218
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                   187,598,626
<SHARES-COMMON-STOCK>                          386,224
<SHARES-COMMON-PRIOR>                          434,072
<ACCUMULATED-NII-CURRENT>                    8,597,008
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                  (15,697,028)
<ACCUM-APPREC-OR-DEPREC>                   (2,253,899)
<NET-ASSETS>                               178,244,707
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            9,709,098
<OTHER-INCOME>                                       0
<EXPENSES-NET>                             (1,165,756)
<NET-INVESTMENT-INCOME>                      8,543,342
<REALIZED-GAINS-CURRENT>                   (8,369,780)
<APPREC-INCREASE-CURRENT>                  (3,487,234)
<NET-CHANGE-FROM-OPS>                      (3,313,672)
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                        528,487
<NUMBER-OF-SHARES-REDEEMED>                  (576,338)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                       (547,322)
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                          308,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                              1,324,905
<AVERAGE-NET-ASSETS>                         4,343,730
<PER-SHARE-NAV-BEGIN>                            9.880
<PER-SHARE-NII>                                  0.470
<PER-SHARE-GAIN-APPREC>                        (0.660)
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                              9.690
<EXPENSE-RATIO>                                  1.000


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

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 4
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   10-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                      149,029,009
<INVESTMENTS-AT-VALUE>                     146,154,905
<RECEIVABLES>                                2,611,619
<ASSETS-OTHER>                                 131,983
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             148,898,507
<PAYABLE-FOR-SECURITIES>                             0
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      126,109
<TOTAL-LIABILITIES>                            126,109
<SENIOR-EQUITY>                            148,772,398
<PAID-IN-CAPITAL-COMMON>                   146,408,022
<SHARES-COMMON-STOCK>                       13,511,574
<SHARES-COMMON-PRIOR>                       14,300,122
<ACCUMULATED-NII-CURRENT>                    8,560,517
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                     3,325,064
<ACCUM-APPREC-OR-DEPREC>                   (2,871,077)
<NET-ASSETS>                               148,722,398
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            9,175,515
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (669,953)
<NET-INVESTMENT-INCOME>                      8,505,562
<REALIZED-GAINS-CURRENT>                   (1,634,425)
<APPREC-INCREASE-CURRENT>                  (9,396,705)
<NET-CHANGE-FROM-OPS>                      (2,525,568)
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                      2,350,626
<NUMBER-OF-SHARES-REDEEMED>                (3,139,173)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                    (10,886,360)
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                 (1,635,684)
<GROSS-ADVISORY-FEES>                          662,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                670,000
<AVERAGE-NET-ASSETS>                       155,365,501
<PER-SHARE-NAV-BEGIN>                           10.930
<PER-SHARE-NII>                                  0.620
<PER-SHARE-GAIN-APPREC>                        (0.790)
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             10.760
<EXPENSE-RATIO>                                  0.500


</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 4
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   10-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                      149,029,009
<INVESTMENTS-AT-VALUE>                     146,154,905
<RECEIVABLES>                                2,611,619
<ASSETS-OTHER>                                 131,983
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                             148,898,507
<PAYABLE-FOR-SECURITIES>                             0
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      126,109
<TOTAL-LIABILITIES>                            126,109
<SENIOR-EQUITY>                            148,722,398
<PAID-IN-CAPITAL-COMMON>                   146,408,022
<SHARES-COMMON-STOCK>                          311,512
<SHARES-COMMON-PRIOR>                          355,243
<ACCUMULATED-NII-CURRENT>                    8,560,517
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                     3,325,064
<ACCUM-APPREC-OR-DEPREC>                   (2,871,077)
<NET-ASSETS>                               148,722,398
<DIVIDEND-INCOME>                                    0
<INTEREST-INCOME>                            9,175,515
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (669,953)
<NET-INVESTMENT-INCOME>                      8,505,562
<REALIZED-GAINS-CURRENT>                   (1,634,425)
<APPREC-INCREASE-CURRENT>                  (9,396,705)
<NET-CHANGE-FROM-OPS>                      (2,525,568)
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                             0
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                         71,699
<NUMBER-OF-SHARES-REDEEMED>                  (115,430)
<SHARES-REINVESTED>                                  0
<NET-CHANGE-IN-ASSETS>                     (2,991,581)
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                 (1,635,684)
<GROSS-ADVISORY-FEES>                          662,000
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                670,000
<AVERAGE-NET-ASSETS>                       155,365,501
<PER-SHARE-NAV-BEGIN>                            10.92
<PER-SHARE-NII>                                  0.610
<PER-SHARE-GAIN-APPREC>                        (0.790)
<PER-SHARE-DIVIDEND>                             0.000
<PER-SHARE-DISTRIBUTIONS>                        0.000
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             10.740
<EXPENSE-RATIO>                                  0.750


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

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 9
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   10-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                       50,559,578
<INVESTMENTS-AT-VALUE>                      67,350,739
<RECEIVABLES>                                  918,782
<ASSETS-OTHER>                                 321,739
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                              68,591,260
<PAYABLE-FOR-SECURITIES>                        28,504
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      430,931
<TOTAL-LIABILITIES>                            459,435
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                    46,255,693
<SHARES-COMMON-STOCK>                            5,369
<SHARES-COMMON-PRIOR>                            5,777
<ACCUMULATED-NII-CURRENT>                    (571,381)
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                      5,621,319
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                    16,826,194
<NET-ASSETS>                                68,131,825
<DIVIDEND-INCOME>                            1,148,543
<INTEREST-INCOME>                               83,034
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (556,484)
<NET-INVESTMENT-INCOME>                        675,093
<REALIZED-GAINS-CURRENT>                     5,252,577
<APPREC-INCREASE-CURRENT>                    2,373,714
<NET-CHANGE-FROM-OPS>                        8,301,384
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                     (51,926)
<DISTRIBUTIONS-OF-GAINS>                     (291,622)
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                      5,615,353
<NUMBER-OF-SHARES-REDEEMED>                (6,049,687)
<SHARES-REINVESTED>                             26,629
<NET-CHANGE-IN-ASSETS>                       2,557,764
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                      286,597
<OVERDISTRIB-NII-PRIOR>                      (867,449)
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                                0
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                      0
<AVERAGE-NET-ASSETS>                        65,767,889
<PER-SHARE-NAV-BEGIN>                           11.270
<PER-SHARE-NII>                                  0.110
<PER-SHARE-GAIN-APPREC>                          1.280
<PER-SHARE-DIVIDEND>                           (0.010)
<PER-SHARE-DISTRIBUTIONS>                      (0.060)
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             12.590
<EXPENSE-RATIO>                                  0.940


</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 9
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   10-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                       50,559,578
<INVESTMENTS-AT-VALUE>                      67,350,739
<RECEIVABLES>                                  918,782
<ASSETS-OTHER>                                 321,739
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                              68,591,260
<PAYABLE-FOR-SECURITIES>                        28,504
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      430,931
<TOTAL-LIABILITIES>                            459,435
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                    46,255,693
<SHARES-COMMON-STOCK>                               44
<SHARES-COMMON-PRIOR>                               44
<ACCUMULATED-NII-CURRENT>                    (571,381)
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                      5,621,319
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                    16,826,194
<NET-ASSETS>                                68,131,825
<DIVIDEND-INCOME>                            1,148,543
<INTEREST-INCOME>                               83,034
<OTHER-INCOME>                                       0
<EXPENSES-NET>                               (556,484)
<NET-INVESTMENT-INCOME>                        675,093
<REALIZED-GAINS-CURRENT>                     5,252,577
<APPREC-INCREASE-CURRENT>                    2,373,714
<NET-CHANGE-FROM-OPS>                        8,301,384
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                        (339)
<DISTRIBUTIONS-OF-GAINS>                       (2,418)
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                          9,123
<NUMBER-OF-SHARES-REDEEMED>                    (9,931)
<SHARES-REINVESTED>                                228
<NET-CHANGE-IN-ASSETS>                       8,298,516
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                      286,597
<OVERDISTRIB-NII-PRIOR>                      (867,449)
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                                0
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                      0
<AVERAGE-NET-ASSETS>                           536,184
<PER-SHARE-NAV-BEGIN>                           11.220
<PER-SHARE-NII>                                  0.090
<PER-SHARE-GAIN-APPREC>                          1.270
<PER-SHARE-DIVIDEND>                           (0.010)
<PER-SHARE-DISTRIBUTIONS>                      (0.060)
<RETURNS-OF-CAPITAL>                             0.000
<PER-SHARE-NAV-END>                             12.510
<EXPENSE-RATIO>                                  1.190


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

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 10
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
<PERIOD-TYPE>                   10-MOS
<FISCAL-YEAR-END>                          OCT-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               OCT-31-1999
<INVESTMENTS-AT-COST>                       59,439,415
<INVESTMENTS-AT-VALUE>                      57,355,936
<RECEIVABLES>                                2,438,643
<ASSETS-OTHER>                                 487,410
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</TABLE>

<TABLE> <S> <C>

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<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
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   <NUMBER> 10
   <NAME> INSTITUTIONAL SERVICE CLASS
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<S>                             <C>
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</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 11
   <NAME> INSTITUTIONAL SERVICE CLASS
<MULTIPLIER> 1

<S>                             <C>
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<DISTRIBUTIONS-OF-INCOME>                      (1,740)
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<PER-SHARE-NAV-END>                             12.970
<EXPENSE-RATIO>                                  1.260


</TABLE>

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 868307
<NAME> MAINSTAY INSTITUTIONAL FUNDS INC.
<SERIES>
   <NUMBER> 11
   <NAME> INSTITUTIONAL CLASS
<MULTIPLIER> 1

<S>                             <C>
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<SENIOR-EQUITY>                                      0
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<PER-SHARE-NAV-END>                             13.100
<EXPENSE-RATIO>                                  1.010


</TABLE>


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