<TABLE> <S> <C>

<PAGE>
<ARTICLE> 5
<LEGEND>
THIS SCHEDULE CONTAINS SUMMARY FINANCIAL INFORMATION EXTRACTED FROM THE
FINANCIAL STATEMENTS OF ESI AND IS QUALIFIED IN ITS ENTIRETY BY REFERENCE TO
SUCH FINANCIAL STATEMENTS.
</LEGEND>
<RESTATED>

<S>                             <C>                     <C>                     <C>                     <C>
<C>
<PERIOD-TYPE>                    YEAR                    YEAR                    3-MOS                   3-MOS
3-MOS
<FISCAL-YEAR-END>                          DEC-31-1996             DEC-31-1995             MAR-31-1997             JUN-30-1997
             SEP-30-1997
<PERIOD-START>                             JAN-01-1996             JAN-01-1995             JAN-01-1997             APR-01-1997
             JUL-01-1997
<PERIOD-END>                               DEC-31-1996             DEC-31-1995             MAR-31-1997             JUN-30-1997
             SEP-30-1997
<CASH>                                       6,157,491               1,863,788              29,451,066              27,198,404
               5,971,430
<SECURITIES>                                         0                       0                       0                       0
              19,153,144
<RECEIVABLES>                                        0                       0                       0                       0
                       0
<ALLOWANCES>                                         0                       0                       0                       0
                       0
<INVENTORY>                                          0                       0                       0                       0
                       0
<CURRENT-ASSETS>                             6,232,544               1,935,999              29,627,888              27,361,371
              25,446,135
<PP&E>                                       1,496,404                 944,136               1,844,815               2,079,967
               2,320,145
<DEPRECIATION>                               (656,695)               (381,067)               (749,053)               (861,139)
               (973,955)
<TOTAL-ASSETS>                               7,200,126               2,594,953              30,847,348              28,703,201
              26,910,920
<CURRENT-LIABILITIES>                          683,577                 518,519                 986,701               1,121,160
               1,370,269
<BONDS>                                              0                       0                       0                       0
                       0
<PREFERRED-MANDATORY>                                0                       0                       0                       0
                       0
<PREFERRED>                                     94,112                  74,575                       0                       0
                       0
<COMMON>                                        10,534                  10,286                  87,850                  88,148
                  88,980
<OTHER-SE>                                  23,444,359              11,973,840                       0                       0
                       0
<TOTAL-LIABILITY-AND-EQUITY>                 7,200,126               2,594,953              30,847,348              28,703,201
              26,910,920
<SALES>                                              0                       0                       0                       0
                       0
<TOTAL-REVENUES>                                     0                       0                       0                       0
                       0
<CGS>                                                0                       0                       0                       0
                       0
<TOTAL-COSTS>                                        0                       0                       0                       0
                       0
<OTHER-EXPENSES>                             6,709,495               4,849,621               1,926,487               2,447,446
               2,539,478
<LOSS-PROVISION>                                     0                       0                       0                       0
                       0
<INTEREST-EXPENSE>                              64,691                  82,993                  78,312                  22,280
                  23,943
<INCOME-PRETAX>                            (6,480,601)             (4,733,736)             (1,865,264)             (2,082,568)
             (2,182,974)
<INCOME-TAX>                                         0                       0                       0                       0
                       0
<INCOME-CONTINUING>                                  0                       0                       0                       0
                       0
<DISCONTINUED>                                       0                       0                       0                       0
                       0
<EXTRAORDINARY>                                      0                       0                       0                       0
                       0
<CHANGES>                                            0                       0                       0                       0
                       0
<NET-INCOME>                               (6,480,601)             (4,733,736)             (1,865,264)             (2,082,568)
             (2,182,974)
<EPS-PRIMARY>                                   (6.30)                  (4.63)                 ($1.09)                  ($.40)
                  ($.34)
<EPS-DILUTED>                                   (6.30)                  (4.63)                 ($1.09)                  ($.40)
                  ($.34)


</TABLE>