EXHIBIT 5.1

                 [HELLER EHRMAN WHITE & MCAULIFFE LETTERHEAD]



                              December 14, 1999


Maxygen, Inc.
515 Galveston Drive
Redwood City, California  94063

                      Registration Statement on Form S-1
                      ----------------------------------

     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
(Registration No. 333-89413) filed with the Securities and Exchange Commission
on October 20, 1999 (as may be further amended or supplemented, the
"Registration Statement") for the purpose of registering under the Securities
Act of 1933, as amended, 6,325,000 shares of its authorized but unissued Common
Stock, par value $.0001 per share (the "Shares").  The Shares, which include up
to 825,000 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 and Invemed Associates, 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 filed
     as an exhibit to the Registration Statement and to be filed with
     the Delaware Secretary of State in connection with the sale of the Shares
     and certified to us by an officer of the Company as being the form to be
     filed with the Delaware Secretary of State in connection with the sale of
     the Shares;

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



Maxygen, Inc.
December 14, 1999



(d)  The Registration Statement; and

(e)  The draft of the Underwriting Agreement filed as Exhibit 1.1 to the
     Registration Statement.

     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.  Our opinion to
the effect that all issued and outstanding Shares are fully paid and
nonassessable is based on the certification obtained from the Company identified
in item (c) above to the effect that the consideration of such Shares recited in
the Board of Directors' resolutions for such Shares has been received.

     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