EXHIBIT 5.1

                                November 26, 2002

Sontra Medical Corporation
58 Charles Street
Cambridge, MA 02141

         Re:   Registration Statement on Form S-8 Relating to Sontra Medical
               Corporation 401(k) Retirement Plan (the "Plan")

Dear Sir or Madam:

         Reference is made to the above-captioned Registration Statement on Form
S-8 (the "Registration Statement") filed by Sontra Medical Corporation (the
"Company") on the date hereof with the Securities and Exchange Commission under
the Securities Act of 1933 relating to an aggregate of 500,000 additional shares
of Common Stock, par value $0.01 per share, of the Company issuable pursuant to
the Plan (the "Shares").

         We have examined, are familiar with, and have relied as to factual
matters solely upon, a copy of the Plan, the Company's Articles of
Incorporation, as amended, the Company's By-Laws, the minute books and stock
records of the Company, and originals or certified copies of such other
certificates, documents, records and materials as we have deemed necessary for
the purpose of rendering this opinion.

         Based upon the foregoing, we are of the opinion that the Shares have
been duly authorized and, when issued in accordance with the terms of the Plan,
will be validly issued, fully paid and nonassessable.

         We hereby consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement.


                                            Very truly yours,

                                            /s/ TESTA HURWITZ & THIBEAULT, LLP

                                            TESTA HURWITZ & THIBEAULT, LLP