EXHIBIT 99.1 CERTIFICATION PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002 (SUBSECTIONS (a) AND (b) OF SECTION 1350, CHAPTER 63 OF TITLE 18, UNITED STATES CODE) Pursuant to section 906 of the Sarbanes-Oxley Act of 2002 (subsections (a) and (b) of section 1350, chapter 63 of title 18, United States Code), each of the undersigned officers of Biogen, Inc., a Massachusetts corporation (the "Company"), does hereby certify, to such officer's knowledge, that: The Annual Report on Form 10-K for the fiscal year ended December 31, 2002 (the "Form 10-K") of the Company fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934, and the information contained in the Form 10-K fairly presents, in all material respects, the financial condition and results of operations of the Company. Dated: March 14, 2003 /s/ James C. Mullen -------------------------------------------- JAMES C. MULLEN CHAIRMAN, CHIEF EXECUTIVE OFFICER AND PRESIDENT Dated: March 14, 2003 /s/ Peter N. Kellogg -------------------------------------------- PETER N. KELLOGG EXECUTIVE VICE PRESIDENT -- FINANCE AND CHIEF FINANCIAL OFFICER The foregoing certification is being furnished solely pursuant to section 906 of the Sarbanes-Oxley Act of 2002 (subsections (a) and (b) of section 1350, chapter 63 of title 18, United States Code) and is not being filed as part of the Form 10-K or as a separate disclosure document. 5