Exhibit 5.1

                                Bingham Dana LLP
                               150 Federal Street
                                Boston, MA 02110


November 13, 2001


Essential Therapeutics, Inc.
1365 Main Street
Waltham, MA  02451

         Re:   Registration Statement on Form S-8 under the Securities Act
               of 1933, as amended

Ladies and Gentlemen:

         We have acted as counsel to Essential Therapeutics, Inc., a Delaware
corporation (the "Company"), in connection with the registration under the
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Securities Act of 1933, as amended (the "Act"), of 362,169 shares (the "Shares")
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of the Company's Common Stock, $0.001 par value per share (the "Common Stock"),
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pursuant to a Registration Statement on Form S-8 initially filed by the Company
with the Securities and Exchange Commission on or about November 13, 2001.

         The Shares include shares of Common Stock that may be issued pursuant
to the grant of awards under The Althexis Company, Inc. 1999 Stock Option Plan,
as amended (the "Plan"), which was assumed by the Company pursuant to the
Agreement and Plan of Merger, dated as of July 27, 2001, by and among the
Company, The Althexis Company, Inc. and California MP Acquisition, Inc. (the
"Merger Agreement").
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         As such counsel, we have reviewed the corporate proceedings taken by
the Company with respect to the authorization of the issuance of the Shares. We
have also examined and relied upon originals or copies, certified or otherwise
authenticated to our satisfaction, of such corporate records, documents,
agreements or other instruments of the Company. As to all matters of fact
(including factual conclusions and characterizations and descriptions of
purpose, intention or other state of mind), we have entirely relied upon
certificates of officers of the Company, and have assumed, without independent
inquiry, the accuracy of those certificates.

         We have assumed the genuineness of all signatures, the conformity to
the originals of all documents reviewed by us as copies, the authenticity and
completeness of all original documents reviewed by us in original or copy form
and the legal competence of each individual executing a document. We have also
assumed that the registration requirements




Essential Therapeutics, Inc.
November 13, 2001
Page 2




of the Act and all applicable requirements of state laws regulating the sale of
securities will have been duly satisfied.

         We further assume that any Shares issued pursuant to the Plan will be
issued in accordance with the terms of such Plan.

         This opinion is limited solely to the Delaware General Corporation Law,
as applied by courts located in Delaware, the applicable provisions of the
Delaware Constitution and the reported judicial decisions interpreting those
laws.

         Subject to the foregoing, it is our opinion that the Shares have been
duly authorized and, upon issuance of the Shares in accordance with the terms of
the Plan, will be validly issued, fully paid and non-assessable.

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

                                                 Very truly yours,

                                                 /s/ BINGHAM DANA LLP

                                                 Bingham Dana LLP