[Heller Ehrman White & McAuliffe Letterhead]
                                                                     EXHIBIT 5.1

March 2, 2000


Maxygen, Inc.
515 Galveston Drive
Redwood City, California 94063
                                                                      22800-0001
Re:    Registration Statement on Form S-1


Dear:  Ladies and Gentlemen:

       We have acted as counsel to Maxygen, Inc., a Delaware corporation (the
"Company"), in connection with the Registration Statement on Form S-1 to be
filed with the Securities and Exchange Commission on March 3, 2000 (as may be
further amended or supplemented, the "Registration Statement") for the purpose
of registering under the Securities Act of 1933, as amended, 1,725,000 shares of
its authorized but unissued Common Stock, par value $.0001 per share (the
"Shares"). The Shares, which include up to 225,000 shares of the Company's
Common Stock issuable pursuant to an over-allotment option granted to the
underwriters, are to be sold pursuant to an Underwriting Agreement (the
"Underwriting Agreement") among the Company and Goldman, Sachs & Co., Robertson
Stephens, Invemed Associates and Credit Suisse First Boston, as representatives
of the several underwriters named in Schedule I to the Underwriting Agreement.

       We have assumed the authenticity of all records, documents and
instruments submitted to us as originals, the genuineness of all signatures, the
legal capacity of natural persons and the conformity to the originals of all
records, documents and instruments submitted to us as copies.

       In rendering our opinion, we have examined the following records,
documents and instruments:

  (a)  The Amended and Restated Certificate of Incorporation of the Company,
       certified by the Delaware Secretary of State as of March 2, 2000, and
       certified to us by an officer of the Company as being complete and in
       full force as of the date of this opinion;

  (b)  The Amended and Restated Bylaws of the Company certified to us by an
       officer of the Company as being complete and in full force and effect as
       of the date of this opinion;

  (c)  A Certificate of an officer of the Company (i) attaching records
       certified to us as constituting all records of proceedings and actions of
       the Board of Directors, including any committee


                                                                   Maxygen, Inc.
                                                                   March 2, 2000
                                                                          Page 2

       thereof, and stockholders of the Company relating to the Shares, and the
       Registration Statement, and (ii) certifying as to certain factual
       matters;

  (d)  The Registration Statement; and

  (e)  A letter from ChaseMellon Shareholder Services, L.L.C., dated as of March
       2, 2000, as to the number of shares of the Company's Common Stock that
       were outstanding on March 1, 2000.

       This opinion is limited to the federal law of the United States of
America and the General Corporation Law of the State of Delaware, and we
disclaim any opinion as to the laws of any other jurisdiction. We further
disclaim any opinion as to any other statute, rule, regulation, ordinance, order
or other promulgation of any other jurisdiction or any regional or local
governmental body or as to any related judicial or administrative opinion.

       Based upon the foregoing and our examination of such questions of law as
we have deemed necessary or appropriate for the purpose of this opinion, and
assuming that (i) the Registration Statement becomes and remains effective
during the period when the Shares are offered and sold, (ii) the Underwriting
Agreement signed by the parties thereto conforms in all material respects to the
draft filed as Exhibit 1.1 to the Registration Statement, (iii) the currently
unissued Shares to be sold by the Company are issued, delivered and paid for in
accordance with the terms of the Underwriting Agreement, (iv) appropriate
certificates evidencing the Shares will be executed and delivered by the
Company, and (v) all applicable securities laws are complied with, it is our
opinion that, when issued by the Company, the currently unissued Shares covered
by the Registration Statement will be legally issued, fully paid and
nonassessable.

       This opinion is rendered to you in connection with the Registration
Statement and is solely for your benefit. This opinion may not be relied upon by
you for any other purpose, or relied upon by any other person, firm, corporation
or other entity for any purpose, without our prior written consent. We disclaim
any obligation to advise you of any change of law that occurs, or any facts of
which we may become aware, after the date of this opinion.

       We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement.

                                        Very truly yours,



                                        /s/ Heller Ehrman White & McAuliffe