Exhibit 31.1

CERTIFICATION:

I, David M. Mott, certify that:

|1. |  | I have reviewed this annual report on Form 10-K/A for the year ended December 31, 2003 of MedImmune, Inc. (the Registrant);
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|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;
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|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;
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|4. |  | The Registrants other certifying officers 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:
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|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;
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|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
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|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 that has materially affected, or is reasonably likely to
                    materially affect, the Registrants internal control over financial
                    reporting; and
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|5. |  | The Registrants other certifying officers 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:
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|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
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|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.
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Date: August 5, 2004

| |  | /s/ David M. Mott
David M. MottVice Chairman and Chief Executive Officer
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