EXHIBIT 5.1 July 15, 2002 Sontra Medical Corporation 58 Charles Street Cambridge, MA 02141 Re: Registration Statement on Form S-8 Relating to each of the 1997 Long-Term Incentive and Stock Option Plan and the Sontra Medical, Inc. 1999 Stock Option and Incentive Plan (together, the "Plans") 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 2,366,539 additional shares of Common Stock, par value $0.01 per share, of the Company issuable pursuant to the Plans (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 and paid for 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