**Exhibit 31.1**

 

**CERTIFICATIONS**

 

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

 

1.             I have reviewed this quarterlyreport on Form 10-Q of Abgenix, Inc.;

 

2.             Based on my knowledge, thisquarterly report does not contain any untrue statement of a material fact or omitto state a material 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, thefinancial statements, and other financial information included in thisquarterly 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 quarterly report;

 

4.             The registrant’s other certifyingofficer and I are responsible for establishing and maintaining disclosurecontrols and procedures (as defined in Exchange Act Rules 13a-15(e) and15d-15(e)) for the registrant and we have:

 

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

 

b) evaluated theeffectiveness of the registrant’s disclosure controls and procedures andpresented in this quarterly report our conclusions about the effectiveness ofthe disclosure controls and procedures, as of the end of the period covered bythis quarterly report based on such evaluation; and

 

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

 

5.             The registrant’s other certifyingofficer and I have disclosed, based on our most recent evaluation of internalcontrol over financial reporting, to the registrant’s auditors and the auditcommittee of registrant’s board of directors (or persons performing theequivalent 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) any fraud, whether ornot material, that involves management or other employees who have asignificant role in the registrant’s internal control over financial reporting

 
| Dated:  August 14, 2003 |                                   |
|-------------------------|----------------------------------------|
|                    |                                   |
|                    |                                   |
|                    |  /s/   RAYMOND M. WITHY      |   |
|                    | Raymond M. Withy, Ph.D.                |
|                    | President  and Chief Executive Officer |
|                    | (Principal  Executive Officer)         |

 

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