Exhibit 99.1

                    CERTIFICATION OF CEO AND CFO PURSUANT TO
                             18 U.S.C. SECTION 1350,
                             AS ADOPTED PURSUANT TO
                  SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

In connection with the Quarterly Report of Regeneron Pharmaceuticals, Inc. (the
"Company") on Form 10-Q for the quarterly period ending March 31, 2003 as filed
with the Securities and Exchange Commission on the date hereof (the "Report"),
Leonard S. Schleifer, M.D., Ph.D., as Chief Executive Officer of the Company,
and Murray A. Goldberg, as Chief Financial Officer of the Company, each hereby
certifies, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section
906 of the Sarbanes-Oxley Act of 2002, to the best of his knowledge, that:

         (1)      The Report fully complies with the requirements of section
13(a) or 15(d) of the Securities Exchange Act of 1934; and

         (2)      The information contained in the Report fairly presents, in
all material respects, the financial condition and result of operations of the
Company.

/s/ Leonard S. Schleifer
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Leonard S. Schleifer, M.D., Ph.D.
Chief Executive Officer
May 15, 2003

/s/ Murray A. Goldberg
- ----------------------------------
Murray A. Goldberg
Chief Financial Officer
May 15, 2003

This certification accompanies the Report pursuant to Section 906 of the
Sarbanes-Oxley Act of 2002 and shall not, except to the extent required by the
Sarbanes-Oxley Act of 2002, be deemed filed by the Company for purposes of
Section 18 of the Securities Exchange Act of 1934, as amended.