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                                  EXHIBIT 31.1

              CERTIFICATION OF CHIEF EXECUTIVE OFFICER PURSUANT TO
                 SECTION 302 OF THE SARBANES-OXLEY ACT OF 2002)

I, James C. Mullen, certify that:

1.       I have reviewed this quarterly report of Biogen, Inc.;

2.       Based on my knowledge, this report does not contain any untrue
         statement of a material fact or omit to state a material fact necessary
         to make the statements made, in light of the circumstances under which
         such statements were made, not misleading with respect to the period
         covered by this report;

3.       Based on my knowledge, the financial statements, and other financial
         information included in this report, fairly present in all material
         respects the financial condition, results of operations and cash flows
         of the registrant as of, and for, the periods presented in this report;

4.       The registrant's other certifying officer(s) and I are responsible for
         establishing and maintaining disclosure controls and procedures (as
         defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) for the
         registrant and have:

                  a) designed such disclosure controls and procedures, or caused
                        such disclosure controls and procedures to be designed
                        under our supervision, to ensure that material
                        information relating to the registrant, including its
                        consolidated subsidiaries, is made known to us by others
                        within those entities, particularly during the period in
                        which this report is being prepared;

                  b) evaluated the effectiveness of the registrant's disclosure
                        controls and procedures and presented in this report our
                        conclusions about the effectiveness of the disclosure
                        controls and procedures, as of the end of the period
                        covered by this report based on such evaluation; and

                  c) disclosed in this report any change in the registrant's
                        internal control over financial reporting that occurred
                        during the registrant's most recent fiscal quarter (the
                        registrant's fourth fiscal quarter in the case of an
                        annual report) that has materially affected, or is
                        reasonably likely to materially affect, the registrant's
                        internal control over financial reporting; and

5.       The registrant's other certifying officer(s) and I have disclosed,
         based on our most recent evaluation of internal control over financial
         reporting, to the registrant's auditors and the audit committee of the
         registrant's board of directors (or persons performing the equivalent
         functions):

                  a) all significant deficiencies and material weaknesses in the
                        design or operation of internal control over financial
                        reporting which are reasonably likely to adversely
                        affect the registrant's ability to record, process,
                        summarize and report financial information; and

                  b) any fraud, whether or not material, that involves
                        management or other employees who have a significant
                        role in the registrant's internal control over financial
                        reporting.

                                           Date: October 28, 2003

                                           /s/ James C. Mullen
                                           -------------------------------------
                                           James C. Mullen
                                           Chairman, Chief Executive Officer
                                            and President