EXHIBIT 5.1

                       [O'MELVENY & MYERS LLP LETTERHEAD]

                               November 13, 2000

Abgenix, Inc.
7601 Dumbarton Circle
Fremont, CA 94555

    Re: Registration Statement on Form S-1

Ladies and Gentlemen:

    This opinion is delivered in our capacity as special counsel to Abgenix,
Inc., a Delaware corporation (the "Company"), in connection with the Company's
registration statement on Form S-1 (the "Registration Statement") to be filed on
or around November 13, 2000, with the Securities and Exchange Commission
relating to the registration under the Securities Act of 1933, as amended (the
"Securities Act"), of 1,500,000 shares of common stock of the Company (the
"Shares").

    In connection with rendering this opinion, we have examined the Amended and
Restated Certificate of Incorporation, as amended to the date hereof and on file
with the Secretary of State of the State of Delaware, the Amended and Restated
Bylaws of the Company, such records of corporate proceedings of the Company as
we deem appropriate for the purposes of this opinion, and the Registration
Statement and the exhibits thereto. We have assumed the genuineness of all
signatures, the authenticity of all documents submitted to us as originals and
the conformity with the originals of all documents submitted to us as copies.

    We express no opinion concerning the laws of any jurisdictions other than
the laws of the United States of America and the General Corporation Law of the
State of Delaware, including the statutory provisions and also all applicable
provisions of the Delaware Constitution and reported judicial decisions
interpreting such laws.

    Based upon the foregoing, we are of the opinion that, when issued as
described in the Registration Statement and in a manner that is consistent with
the Authorizing Resolution, the Shares will be legally issued, fully paid and
nonassessable.

    The foregoing assumes that all requisite steps were and will be taken to
comply with the requirements of the Securities Act and all applicable
requirements of state laws regulating the offer and sale of the Securities.

    We hereby consent to the references in the Registration Statement to our
firm under the caption "Legal Matters" and to the inclusion of this opinion as
an exhibit to the Registration Statement. In giving this consent, we do not
admit that we are in the category of persons whose consent is required by
Section 7 of the Securities Act or the rules and regulations promulgated
thereunder by the SEC.

                                         Respectfully yours,
                                          /s/ O'MELVENY & MYERS LLP