EXHIBIT 32.1

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

     In connection with Amendment No. 1 to the Quarterly Report on Form 10-Q of
ImClone Systems Incorporated (the "Company") for the period ended September 30,
2003 as filed with the Securities and Exchange Commission on the date hereof
(the "Report"), I, Daniel S. Lynch, Acting Chief Executive Officer of the
Company, certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to
Section 906 of the Sarbanes-Oxley Act of 2002, 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 results of operations of the
     Company.

                                                  /s/ DANIEL S. LYNCH
                                          --------------------------------------
                                                     Daniel S. Lynch
                                              Acting Chief Executive Officer

November 13, 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.

     A signed original of this written statement required by Section 906 has
been provided to the Company and will be retained by the Company and furnished
to the Securities and Exchange Commission or its staff upon request.