**Exhibit 31.1**

**CERTIFICATION:**

I, David M. Mott,certify that:

1.                I have reviewedthis annual report on Form 10-K of MedImmune, Inc.

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

3.                Based on myknowledge, the financial statements, and other financial information includedin this annual report, fairly present in all material respects the financialcondition, results of operations and cash flows of the registrant as of, andfor, the periods presented in this annual report;

4.                The registrant’sother certifying officer and I are responsible for establishing and maintainingdisclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and15d-15(e)) and internal controlover financial reporting (as defined in Exchange Act Rules 13a-15(f) and15d-15(f)) for the registrant and have:

(a)           designed suchdisclosure controls and procedures or caused such disclosure controls andprocedures to be designed under our supervisory to ensure that materialinformation relating to the registrant, including its consolidatedsubsidiaries, is made known to us by others within those entities, particularlyduring the period in which this annual report is being prepared;

(b)          designed such internalcontrol over financial reporting, or caused such internal control overfinancial reporting to be designed under our supervision, to provide reasonableassurance regarding the reliability of financial reporting and the preparationof financial statements for external purposed in accordance with generallyaccepting accounting principles;

(c)           evaluated theeffectiveness of the registrant’s disclosure controls and procedures as of theend of the period covered by this report based on such evaluation; and

(d)          disclosed in this reportany change in the registrant’s internal control over financial reporting thatoccurred during the registrant’s most recent fiscal quarter (the registrant’sfourth fiscal quarter in the case of an annual report) that has materiallyaffected, or is reasonably likely to materially affect, the registrant’sinternal control over financial reporting; and

5.                The registrant’sother certifying officer and I have disclosed, based on our most recentevaluation of internal control over financial reporting, to the registrant’sauditors and the audit committee of the registrant’s board of directors (orpersons performing the equivalent functions):

(a)           all significantdeficiencies and material weaknesses in the design or operation of internalcontrol over financial reporting which are reasonably likely to adverselyaffect the registrant’s ability to record, process, summarize and reportfinancial information; and

(b)          anyfraud, whether or not material, that involves management or other employees whohave a significant role in the registrant’s internal control over financialreporting.
| Date: March 7, 2005 | /s/ DAVID M.  MOTT                       |
|--------------------------|------------------------------------------|
|                     | David M. Mott                            |
|                     | *Chief Executive Officer, President and* |
|                     | *Vice Chairman of the Board*             |

 

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