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                                   EXHIBIT 5.1



December 22, 1999


Cerus Corporation
2525 Stanwell Drive, Suite 300
Concord, CA  94520



Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection
with the filing by Cerus Corporation (the "Company") of a Registration Statement
on Form S-3 (the "Registration Statement") with the Securities and Exchange
Commission covering the offering for resale of 1,000,000 shares of the Company's
Common Stock (the "Shares"), with a par value of $0.001, issued in connection
with that certain Purchase Agreement by and between the Company and the
purchasers named therein.

In connection with this opinion, we have examined the Registration Statement,
the Company's Certificate of Incorporation and Bylaws, as amended, the
resolutions adopted by the Board of Directors of the Company on November 18 and
December 17, 1999, and such other documents, records, certificates, memoranda
and other instruments as we deem necessary as a basis for this opinion. We have
assumed the genuineness and authenticity of all documents submitted to us as
originals, the conformity to originals of all documents submitted to us as
copies thereof, and the due execution and delivery of all documents where due
execution and delivery are a prerequisite to the effectiveness thereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion
that the Shares, when issued and sold in accordance with the Purchase Agreement,
will be validly issued, fully paid and nonassessable.

We consent to the filing of this opinion as an exhibit to the Registration
Statement and to the reference to our firm under the caption "Legal Matters" in
the Prospectus included in the Registration Statement.



Sincerely,



Andrea Vachss