**Exhibit 31.1**

 

**CERTIFICATIONS**

 

I,Raymond M. Withy, Ph.D., certify that:

 

1.                                       I have reviewed this quarterly report onForm 10-Q of Abgenix, Inc.;

 

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

 

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

 

4.                                       The registrant’s other certifying officer andI are responsible for establishing and maintaining disclosure controls andprocedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) forthe registrant and we have:

 

a)designed such disclosure controls and procedures, or caused such disclosurecontrols and procedures to be designed under our supervision, to ensure thatmaterial information relating to the registrant, including its consolidatedsubsidiaries, is made known to us by others within those entities, particularlyduring the period in which this quarterly report is being prepared;

 

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

 

c)disclosed in this quarterly report any change in the registrant’s internalcontrol over financial reporting that occurred during the registrant’s mostrecent fiscal quarter (the registrant’s fourth fiscal quarter in the case of anannual report) that has materially affected, or is reasonably likely tomaterially affect, the registrant’s internal control over financial reporting;and

 

5.                                       The registrant’s other certifying officer andI have disclosed, based on our most recent evaluation of internal control overfinancial reporting, to the registrant’s auditors and the audit committee ofregistrant’s board of directors (or persons performing the equivalentfunctions):

 

a)all significant deficiencies and material weaknesses in the design or operationof internal control over financial reporting which are reasonably likely toadversely affect the registrant’s ability to record, process, summarize andreport financial information; and

 

b)any fraud, whether or not material, that involves management or other employeeswho have a significant role in the registrant’s internal control over financialreporting.

 

 
| Dated:  May 10, 2004 |                                              |
|----------------------|---------------------------------------------------|
|                 |                                              |
|                 |                                              |
|                 |   /s/   RAYMOND M. WITHY |   |
|                 | Raymond M. Withy, Ph.D.                           |
|                 | President and Chief Executive Officer             |
|                 | (Principal Executive Officer)                     |

 

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