Document:

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                                                                   Exhibit 10.09

                           SONTRA MEDICAL CORPORATION

                     AGREEMENT TO AMEND EMPLOYMENT AGREEMENT
                 AND ENTER INTO INDEPENDENT CONTRACTOR AGREEMENT

         THIS AGREEMENT TO AMEND EMPLOYMENT AGREEMENT AND ENTER INTO INDEPENDENT
CONTRACTOR AGREEMENT ("Agreement") dated November 1, 2002 (the "Effective Date")
is made between Sontra Medical Corporation and its affiliates, successors,
assigns and duly authorized representatives (the "Company") and Joseph Kost (the
"Contractor"), for the purpose of setting forth the exclusive terms and
conditions by which the Company and Contractor desire to (i) amend that certain
Employment Agreement dated June 12, 2001 entered into by and between the Company
and Joseph Kost (the "Employment Agreement") and (ii) acquire Contractor's
services on a temporary basis.

         In consideration of the mutual obligations specified in this Agreement,
the parties, intending to be legally bound hereby, agree to the following:

         1.       Amendment of Employment Agreement: The parties hereby amend
the Employment Agreement by deleting paragraph 3.1 of the Employment Agreement
and replacing it with the following:

                  "3.1. Base Salary. The Executive's salary, effective as of
         November 1, 2002, shall be paid at a rate of $10,000.00 per month
         (annualized compensation of $120,000.00) (the "Base Salary"). The Base
         Salary shall be payable in accordance with the customary payroll
         practices of the Company as may be established or modified from time to
         time. The Board of Directors in its sole discretion may adjust the
         Executive's salary at any time. All payments shall be subject to all
         applicable federal, state and/or local payroll and withholding taxes."

         Except as expressly set forth above, all terms and conditions of the
Employment Agreement shall remain in full force and effect until January 1,
2003, at which time the parties expressly agree to rescind the Employment
Agreement in its entirety. As such, as of January 1, 2003, the parties agree
that the Employment Agreement will be of no further force and effect. In
rescinding the Employment Agreement, the parties acknowledge and agree that the
terms and conditions of the Employment Agreement and all obligations arising
from such terms and conditions, including any obligations intended to survive
the termination of such Employment Agreement, will no longer be enforceable as
of January 1, 2003. The parties further agree, however, that effective January
1, 2003, Joseph Kost shall commence providing Services (as defined herein) to
the Company pursuant to the remainder of this Agreement as an Independent
Contractor. As such, all remaining provisions of this Agreement are effective on
January 1, 2003.

         2.       Services:

         (a)      Company hereby retains Contractor, and Contractor hereby
agrees to perform for Company, certain services assigned to Contractor by the
Company in the Company's sole discretion, including, but not limited to,
rendering scientific advice and counsel to the Company in the

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"Consulting Field", wherein "Consulting Field" is defined as any and all
applications in ultrasound enhanced diagnostics and transdermal drug delivery
(collectively the "Services"):

         (b)      Contractor agrees to keep Company updated, promptly upon
Company's request, of any progress, problems, and/or developments of which
Contractor is aware regarding the Services. Company shall have the right to
require such updates in writing from Contractor in a format specified by Company
or acceptable to Company in its sole discretion.

         3.       Consideration/Compensation:

         (a)      In exchange for the full, prompt, and satisfactory performance
of all Services to be rendered to Company hereunder (as determined by the
Company), Company shall provide Contractor, as full and complete compensation
for the Services rendered hereunder, compensation consisting of (i) cash at a
rate of $10,000.00 per month (annualized compensation of $120,000.00), unless
and until directed otherwise in writing by the Chief Executive Officer of the
Company. Contractor shall invoice Company on a monthly basis setting forth in
detail the Services performed by Contractor. Company shall remit payment to
Contractor within a reasonable time thereafter. Failure by Contractor to submit
a detailed invoice shall be grounds for Company to withhold any and all
compensation.

         (b)      Contractor shall not be entitled to receive any other
compensation or any benefits from Company. Except as otherwise required by law,
Company shall not withhold any sums or payments made to Contractor for social
security or other federal, state or local tax liabilities or contributions, and
all withholdings, liabilities, and contributions shall be solely Contractor's
responsibility. Further, Contractor understands and agrees that the Services are
not covered under the unemployment compensation laws and are not intended to be
covered by workers' compensation laws.

         4.       Nondisclosure:

         (a)      Contractor understands that, in connection with his engagement
with Company, he may receive, produce, or otherwise be exposed to Company's
Confidential Information, which is defined as any information concerning the
organization, business or finances of the Company or of any third party which
the Company is under an obligation to keep confidential and that is maintained
by the Company as confidential. Such Confidential Information shall include, but
is not limited to, trade secrets or proprietary or confidential information
respecting existing and future products and services, inventions, designs,
methods, formulas, drafts of publications, research, know-how, techniques,
systems, databases, processes, software programs or code, developments or
experimental work, works of authorship, customer lists and/or customer
information, business plans, marketing plans, financial information, sales
techniques, projects, the Company's salary and/or pay rates, other Company
personnel information, and all other plans and proposals.

         (b)      Contractor acknowledges that the Confidential Information is
the Company's sole, exclusive and extremely valuable property. Accordingly,
Contractor agrees to segregate all Confidential Information from information of
other companies and agrees not to reproduce any Confidential Information without
Company's prior written consent, not to use the Confidential Information except
in the performance of this Agreement, and not to divulge all or any part of the
Confidential Information in any form to any third party, either during or after
the term of this

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Agreement, except to Company employees and Contractor Employees, if any, who
need to know such Confidential Information in order to perform the Services.
Contractor shall require such Contractor Employees to execute a nondisclosure
agreement satisfactory to the Company before such Contractor Employee is exposed
to any Confidential Information. Upon termination or expiration of this
Agreement for any reason, Contractor agrees to cease using and to return to
Company all whole and partial copies and derivatives of the Confidential
Information, whether in Contractor's possession or under Contractor's direct or
indirect control, including any computer access nodes and/or codes, and to
arrange for the return of such materials by all Contractor Employees.

         (c)      Contractor shall not disclose or otherwise make available to
Company in any manner any confidential and proprietary information received by
Contractor from third parties. Contractor warrants that his performance of all
the terms of this Agreement does not and will not breach any agreement entered
into by Contractor with any other party, and Contractor agrees not to enter into
any agreement, oral or written, in conflict herewith. In addition, Contractor
recognizes that the Company has received and in the future will receive from
third parties their confidential or proprietary information subject to a duty on
the Company's part to maintain the confidentiality of such information and to
use such information only for certain limited purposes. Contractor agrees that
he owes the Company and such third parties, during the term of the Contractor's
relationship with the Company and thereafter, regardless of the reason for the
termination of the relationship, a duty to hold all such confidential or
proprietary information in the strictest of confidence and not to disclose such
information to any person, firm or corporation (except as necessary in carrying
out his work for the Company consistent with the Company's agreement with such
third party) or to use such information for the benefit of anyone other than for
the Company or such third party (consistent with the Company's agreement with
such third party).

         (d)      Contractor also agrees that during his consultancy with the
Company that he shall not make, use or permit to be used any Company Property
otherwise than for the benefit of the Company. The term "Company Property" shall
include all notes, memoranda, reports, lists, records, drawings, sketches,
designs, specifications, software programs, software code, data, computers, palm
pilots and their equivalents, cellular telephones, pagers, credit and/or calling
cards, keys, access cards, documentation or other materials of any nature and in
any form, whether written, printed, electronic or in digital format or
otherwise, relating to any matter within the scope of the business of the
Company or concerning any of its dealings or affairs, and any other Company
property in his possession, custody or control. Contractor further agrees that
he shall not, after the termination of his consultancy, use or permit others to
use any such Company Property. Contractor acknowledges and agrees that all
Company Property shall be and remain the sole and exclusive property of the
Company. Immediately upon the termination of Contractor's consultancy,
Contractor shall deliver all Company Property in his possession, and all copies
thereof, to the Company.

         5.       Ownership of Work Product:

         (a)      Contractor agrees to promptly disclose to Company any and all
Work Product. "Work Product" includes without limitation any and all notes,
drawings, designs, technical data, know how, works of authorship, firmware,
software, ideas, improvements, inventions, material, information, work or
product conceived, created, written or first reduced to practice by Contractor

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either solely or jointly with others in the performance of consulting services
for Company and/or resulting from use of Confidential Information by Contractor
solely or jointly with others. Contractor agrees to assign and does hereby
assign to Company his entire right, title and interest, including without
limitation any copyright, patent, trade secret, trademark (including the good
will associated therewith) or other intellectual property rights in and to the
Work Product. All works of authorship, firmware, software or other applicable
Work Product shall be considered works for hire by Contractor for Company and
all Work Product shall be the Company's sole and exclusive property. Contractor
agrees to and shall provide Company with all Work Product generated on Company's
behalf.

         (b)      Contractor hereby grants to Company under any and all
intellectual property rights a non-exclusive, irrevocable, royalty free, and
worldwide license to use all notes, works of authorship, ideas, designs,
firmware, software, technical data, ideas, research, reports, and other such
inventions which are not Work Product, but which are owned by Contractor and are
necessary or desirable to use any Work Product delivered by Contractor to
Company (individually and collectively, "Background Material") including,
without limitation, the right to make, have made, sell, offer for sale, rent,
lease, import, copy, create derivative works, display, perform, and distribute
the Background Material.

         (c)      Contractor further agrees:

                  (i)      to apply for, obtain, register and vest in the name
                           of the Company alone (unless the Company otherwise
                           directs) patents, copyrights, trademarks or other
                           analogous protection in any country throughout the
                           world and when so obtained or vested to renew and
                           restore the same; and

                  (ii)     to defend any judicial, opposition or other
                           proceedings in respect of such applications and any
                           judicial, opposition or other proceedings or
                           petitions or applications for revocation of such
                           patent, copyright, trademark or other analogous
                           protection.

         (d)      Contractor also agrees, at the request and cost of Company, to
promptly sign, execute, make and do all such deeds, documents, acts and things
as Company may reasonably require or desire to perfect Company's entire right,
title, and interest in and to any Work Product and/or Background Material.
Contractor agrees that if Company is unable because of Contractor's
unavailability, or for any other reason, to secure the signature of an
authorized agent of Contractor to apply for or to pursue any application for any
United States or foreign patents, copyright or trademark registrations covering
the assignments to Company, then Contractor hereby irrevocably designates and
appoints Company and its duly authorized officers and agents as Contractor's
agent and attorney in fact, to act for and in Contractor's behalf and stead to
execute and file any such applications and to do all other lawfully permitted
acts to further the prosecution and issuance of patents, copyright, and
trademark registrations thereon with the same legal force and effect as if
executed by an authorized agent of Contractor.

         (e)      Contractor warrants that it has good and marketable title to
the Background Material and Work Product and that he shall not knowingly
incorporate into any Work Product any material

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that would infringe any copyright, trade secret, trademark or other intellectual
property rights of any person or entity. Contractor further warrants that the
Background Material and Work Product shall be free and clear of all liens,
claims, encumbrances or demands of third parties, including any claims by any
such third parties of any right, title or interest in or to the Background
Material and/or Work Product arising out of any patent, trade secret, copyright
or other intellectual property right. Contractor shall indemnify, defend and
hold harmless the Company and its customers from any and all liability, loss,
cost, damage, judgment or expense (including reasonable attorney's fees)
resulting from or arising in any way out of any such claims by any third
parties, and/or which are based upon, or are the result of any breach of, the
warranties contained in this Section 4. In the event of a breach of the
warranties set forth in this Section 4, in addition to all other remedies
available to Company, Contractor shall, at no additional cost to Company,
replace or modify the Work Product within a reasonable time, with a functionally
equivalent and conforming Work Product at his/her own expense, or obtain for
Company the right to continue using the Work Product and in all other respects
use his/her best efforts to remedy the breach.

         6.       Noncompetition; Nonsolicitation. During the period of this
Agreement and for one year thereafter, Contractor shall not, directly or
indirectly, alone or as a partner, joint venturer, consultant, contractor,
lender, officer, director, employee, shareholder or investor of any entity,
engage in any business or activity within the United States which is in
competition with the products or services which, during such period, are being
or are proposed to be created, developed, manufactured, marketed, distributed or
sold by the Company. Further, during the period of this Agreement and for one
year thereafter, Contractor shall not, directly or indirectly, alone or as a
partner, joint venturer, consultant, contractor, lender, officer, director,
employee, shareholder or investor of any entity, (a) solicit or do business with
any customer of the Company or any potential customer of the Company with whom
Contractor has had contact or about whom he has obtained Confidential
Information, or (b) recruit or solicit for hire any person who is or was an
employee, agent, representative or consultant of the Company at any time during
the term of this Agreement or during the period of one year thereafter, or in
any manner seek to solicit or induce any such person to leave his employment
with the Company, or assist in the recruitment or hiring of any such person.
Contractor's record or beneficial ownership of not more than five percent (5%)
of the outstanding publicly traded capital stock of any entity shall not be
deemed, in and of itself, to be in violation of this Paragraph.

         7.       Indemnification/Release:

         (a)      Contractor agrees to take all necessary precautions to prevent
injury to any persons (including employees of Company) or damage to property
(including Company's property) during the term of this Agreement, and shall
indemnify, defend and hold harmless Company, its officers, directors,
shareholders, employees, representatives and/or agents from any claim,
liability, loss, cost, damage, judgment, settlement or expense (including
attorney's fees) resulting from or arising in any way out of injury (including
death) to any person or damage to property arising in any way out of any act,
error, omission or negligence on the part of Contractor or any Contractor
Employee in the performance or failure to fulfill any Services or obligations
under this Agreement. Contractor further agrees to indemnify, defend and hold
harmless Company, its officers, directors, shareholders, employees,
representatives and/or agents from the costs to Company of remedying any
violation or

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breach of this Agreement by Contractor and/or from the costs to the Company
resulting from a breach by Contractor to a third party during the performance of
the Services under this Agreement.

         (b)      Contractor agrees to indemnify and hold the Company harmless
from and against any and all claims, losses, demands, liabilities, damages,
costs, or expenses (including, without limitation, attorney's fees, back wages,
liquidated damages, penalties or interest) resulting from any violation of any
federal, state or local law, regulation, or ordinance by Contractor, including,
without limitation, Contractor's failure to collect, withhold, or pay any and
all federal or state taxes required to be withheld or paid by employers or
employees, including, without limitation, any and all income tax, social
security, and FUTA taxes.

         (c)      In remedying any violation of this Agreement by Contractor,
the Company shall first notify Contractor of the existence of such violation and
provide Contractor thirty (30) days in which to remedy the violation at
Contractor's expense.

         8.       Termination: This Agreement shall be effective on the date
hereof and shall continue until terminated by either party upon written notice.
In the event of termination, Contractor shall, upon request, perform such work
as may be requested to transfer work in process to the Company or to a party
designated by the Company.

         9.       Independent Contractor:

         (a)      Company and Contractor expressly agree and understand that
Contractor is an independent contractor and nothing in this Agreement nor the
services rendered hereunder is meant, or shall be construed in any way or
manner, to create between them a relationship of employer and employee,
principal and agent, partners or any other relationship other than that of
independent parties contracting with each other solely for the purpose of
carrying out the provisions of this Agreement. Contractor is not the agent of
Company and is not authorized and shall not have the power or authority to bind
Company or incur any liability or obligation, or act on behalf of Company. At no
time shall Contractor represent that he is an agent of the Company, or that any
of the views, advice, statements and/or information that may be provided while
performing the Services are those of the Company.

         (b)      While Company is entitled to provide Contractor with general
guidance to assist Contractor in completing the scope of work to Company's
satisfaction, Contractor is ultimately responsible for directing and controlling
the performance of the task and the scope of work, in accordance with the terms
and conditions of this Agreement. Contractor shall use his best efforts, energy
and skill in his own name and in such manner as he sees fit.

         10.      Contractor Warranties:

         (a)      The execution and performance of this Agreement will not
constitute a breach or default under any contract or instrument to which
Contractor is a party, or by which he is bound, including, without limitation,
any and all employment, noncompetition and nondisclosure agreements with any
former employer or customer, and Contractor is under no contractual or other
obligation to any third party which would prevent or limit his performance or
services under this Agreement.

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         (b)      Contractor has complied with all federal, state, and local
laws regarding business permits and licenses that may be required to carry out
the services to be performed under this Agreement.

         (c)      Contractor is free to disclose to the Company, without breach
of any obligation to a third party, any and all information, ideas, suggestions,
developments, or know-how that Contractor will divulge in performing the
services under this Agreement.

         11.      General:

         (a)      This Agreement does not create an obligation on Company to
continue to retain Contractor beyond this Agreement's termination.

         (b)      This Agreement may not be changed unless mutually agreed upon
in writing by both Contractor and by an authorized officer of the Company.

         (c)      Sections 1, 3, 4, 5, 6, 7, 8, 9 and 10 shall survive the
termination of this Agreement regardless of the manner of such termination.

         (d)      Any waiver by Company of a breach of any provision of this
Agreement shall not operate or be construed as a waiver of any subsequent breach
of such provision or any other provision hereof.

         (e)      Contractor hereby agrees that each provision herein shall be
treated as a separate and independent clause, and the unenforceability of any
one clause shall in no way impair the enforceability of any of the other clauses
herein. Moreover, if one or more of the provisions contained in this Agreement
shall for any reason be held to be excessively broad as to scope, activity,
subject or otherwise so as to be unenforceable at law, such provision or
provisions shall be construed by the appropriate judicial body by limiting or
reducing it or them, so as to be enforceable to the maximum extent compatible
with the applicable law as it shall then appear.

         (f)      Company shall have the right to assign this Agreement to its
successors and assigns, and this Agreement shall inure to the benefit of and be
enforceable by said successors or assigns. Contractor may not assign this
Agreement or any rights or obligations hereunder without the prior written
consent of Company's Chief Executive Officer, and this Agreement shall be
binding upon Contractor's heirs, executors, administrators and legal
representatives.

         (g)      This Agreement and all aspects of the relationship between the
parties hereto shall be construed and enforced in accordance with and governed
by the internal laws of the Commonwealth of Massachusetts without regard to its
conflict of laws provisions. The Contractor further agrees that any claims or
legal actions by one party against the other arising out of the relationship
between the parties contemplated herein (whether or not arising under this
Agreement) shall be commenced and maintained in any state or federal court
located in Massachusetts, and Contractor hereby submits to the jurisdiction and
venue of any such court.

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         (h)      This Agreement sets forth the complete, sole and entire
agreement between the parties with respect to the subject matter herein and
supersedes any and all other agreements, negotiations, discussions, proposals,
or understandings, whether oral or written, previously entered into, discussed
or considered by the parties, including without limitation, the June 1, 1998
Consulting Agreement entered into by and between the parties.

         (i)      The language of all parts of this Agreement will in all cases
be construed as a whole in accordance with its fair meaning and not strictly for
or against either party hereto.

         (j)      Contractor further agrees that any breach of this Agreement by
him will cause irreparable damage to the Company and that in the event of such
breach the Company shall have, in addition to any and all remedies of law, the
right to an injunction, specific performance or other equitable relief to
prevent Contractor from violating his obligations hereunder.

         (k)      All notices provided for in this Agreement shall be given in
writing and shall be effective when either served by hand delivery, express
overnight courier service, or by registered or certified mail, return receipt
requested, addressed to the parties at their respective addresses: for the
Company: 58 Charles Street, Cambridge, MA 02141; for the Contractor: 931
Massachusetts Ave., #30, Cambridge, MA 02139, or to such other address or
addresses as either party may later specify by written notice to the other.

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         IN WITNESS WHEREOF, the parties hereto have executed this Independent
Contractor Agreement.

         JOSEPH KOST:                                SONTRA MEDICAL CORPORATION:

         /s/ Joseph Kost                             By: /s/ Thomas W. Davison
         ------------------                              -----------------------
         Signature
                                                     Title: President and CEO
                                                            --------------------

         Date: 12/22/02                              Date: 12/12/02
               ------------                                ---------------------

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                                                                   EXHIBIT 10.10

                              CONSULTING AGREEMENT

          WHEREAS, effective as of June 1, 1998, SONTRA MEDICAL, INC. (formerly
"Newcorp Medical, Inc."), a corporation organized under the laws of the State of
Delaware and having its principal office at 21 Erie Street, Suite 22, Cambridge,
Massachusetts 02139 (hereinafter referred to as "Sontra"), and Dr. Robert S.
Langer, residing at 77 Lombard Street, Newton, Massachusetts 02158
("Consultant") desire to enter into this Consulting Agreement;

          NOW THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, Sontra and Consultant hereby agree
to such Consulting Agreement, effective as of June 1, 1998, so that it reads in
its entirety as follows:

          Sontra desires to engage Consultant to perform consulting services for
Sontra and Consultant desires to perform such services, on the terms and
conditions hereinafter set forth.

                 Term

          Sontra agrees to retain Consultant, and Consultant agrees to serve, on
the terms and conditions of this Agreement for a period commencing on the
effective date of the License Agreement (the "License Agreement") between Sontra
and Massachusetts Institute of Technology ("M.I.T.") (the "Effective Date") and
ending December 1, 1999, subject to earlier termination as provided herein. The
period during which consultant is engaged hereunder is hereafter referred to as
the "Consulting Period." This Agreement may be renewed by mutual consent of both
parties.

          2.     Duties

          Consultant is hereby engaged to act as a consultant to Sontra to
render scientific advice and counsel to Sontra in the "Consulting Field," herein
defined as all applications in ultrasound enhanced diagnostics and transdermal
drug delivery, as such definition may be modified

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          from time to time pursuant to the provisions of Section 6. Consultant
agrees to lead all projects relating thereto. In connection with rendering such
services, Consultant shall:

          (a)    report to officers, employees and agents of Sontra and other
                 persons or groups at mutually agreeable times and as reasonably
                 requested by Sontra from time to time; interface on behalf of
                 Sontra with the scientific community, peers, Institutional
                 Review Boards, the medical and business communities and other
                 persons or groups at mutually agreeable times and as reasonably
                 requested by Sontra time to time;

          (c)    consult with Sontra employees at mutually agreeable times;

          (d)    participate as a member of Sontra's Board of Scientific
                 Advisors, if any, at mutually agreeable times and as may be
                 requested by Sontra, at no additional compensation to that set
                 forth in Section 3 herein; and engage in such other scientific
                 undertakings at mutually agreeable times and as Consultant and
                 Sontra may agree upon during the Consulting Period.

          Consultant shall devote such time and energies as are reasonably
necessary to fulfill his obligations hereunder. Sontra and Consultant shall both
make good faith attempts to schedule any consulting services so as not to
interfere unreasonably with Consultant's other activities. It is understood that
Consultant shall be an independent contractor who may be engaged in other
employment or who shall render other consulting services during the period of
this Consulting Agreement, subject to the limitations of Section 6 hereof.

          3.     Compensation

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          As compensation for his services hereunder, Sontra shall pay
Consultant a consulting fee, which during the first year of the Consulting
Period shall be at the annual rate of $120,000, and thereafter at a rate as
shall be agreed between the parties hereto, payable in monthly installments on
the last day of each month of each year (commencing June 1, 1998). The payment
of the consulting fee shall be deferred and shall not be paid until the earlier
of (i) a determination by Sontra, in the exercise of its sole discretion, that
there are sufficient Sontra cash resources to make all or any part of such
payment on an equal and ratable basis to all consultants of Sontra whose
consulting fees are then being similarly deferred; and (ii) the dissolution and
termination of Sontra. Consultant acknowledges that as an independent contractor
he is not entitled to participate in or receive any benefit or right offered to
employees of Sontra under any employee benefit plan, including, without
limitation, medical and health insurance. Consultant acknowledges that Sontra
will not withhold taxes on any amounts paid to him hereunder and that Consultant
is responsible for all income taxes, social security taxes, employment taxes,
unemployment taxes and insurance, and other similar payments.

          4.     Expenses

          Consultant shall be entitled to reimbursement for all reasonable
travel and other out-of-pocket expenses necessarily incurred in the performance
of his duties hereunder, upon submission and approval (not to be unreasonably
withheld) of written statements and bills in accordance with the then regular
procedures of Sontra.

          5.     Representations and Warranties of Consultant

          Consultant represents and warrants to Sontra that, to the best of his
knowledge, Consultant is under no contractual or other restriction or obligation
that is inconsistent with the

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execution of this Agreement, the performance of his duties hereunder, or the
other rights of Sontra hereunder. Consultant represents and warrants that he
will work in good faith to carry out his consulting duties.

          6.     Noncompetition

          (a)    During the term of this Agreement and for a period of one year
commencing on the expiration of this Agreement (the "Noncompetition Period"),
Consultant agrees that he will not perform consulting or research services for,
or accept employment with, or acquire (or negotiate with respect to the
acquisition of) control of any entity located anywhere in the world which
engages, proposes to engage, or could be deemed to be engaged in any manner in
the Consulting Field. Consultant represents and warrants to Sontra that listed
on Appendix A hereto are the only activities of the type specified in the first
sentence of this Section 6(a) currently engaged in by Consultant. Sontra agrees
that for purposes of this Agreement the activities listed on Appendix A hereto
do not fall within the Consulting Field. If, during the Noncompetition Period,
Consultant desires to perform consulting or research services for, or accept
employment with, or acquire (or negotiate with respect to the acquisition of)
control of any entity located anywhere in the world which engages, proposes to
engage, or could be deemed to be engaged in any manner in the Consulting Field,
and any such activity is not then listed on Appendix A, then Consultant must
propose, in writing, an amendment to Appendix A disclosing the proposed activity
(the "Proposed Amendment").

          (b)    If an activity disclosed in a Proposed Amendment relates to
patents, patent applications, inventions, technological innovations, copyrights,
copyrightable works, developments, discoveries, designs, processes, formulas,
know-how, data or analysis, whether patentable or not (the "Unlicensed
Technology"), which, as a result of Consultant's pre-existing and on-going
obligations

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to M.I.T., are then owned by M.I.T., then Consultant shall cooperate fully with
Sontra in Sontra's efforts to license such Unlicensed Technology. If Sontra
obtains a license to such Unlicensed Technology within 120 days following the
delivery of the Proposed Amendment, Consultant hereby agrees that the Unlicensed
Technology shall be within the meaning of the Consulting Field for all purposes
of this Agreement, and Consultant shall not be entitled to engage in such
activity during the Noncompetition Period for any person or entity other than
Sontra. If Sontra does not obtain a license to such Unlicensed Technology within
such period (either because Sontra does not attempt to obtain such a license or
because Sontra is unable to negotiate successfully to obtain such a license),
and the failure to obtain a license is not the result of action taken or failed
to be taken by Consultant, then Sontra hereby agrees that the Unlicensed
Technology shall not be within the meaning of Consulting Field for all purposes
of this Agreement, and agrees to an amendment to Appendix A to such effect.

          (c)    If the activity disclosed in a Proposed Amendment does not
relate to Unlicensed Technology which, as a result of Consultant's pre-existing
and on-going obligations to M.I.T., are then owned by M.I.T., and if Sontra
does not agree within 30 days following delivery of the Proposed Amendment that
the proposed new activity falls outside the Consulting Field, and therefore does
not agree to the Proposed Amendment, then Consultant shall not be entitled to
engage in such activity during the Noncompetition Period for any person or
entity other than Sontra.

          (d)    Anything in Sections 6(b) or 6(c) to the contrary
notwithstanding, however, if Consultant disagrees with Sontra's determination
that, as provided in Section 6(b), Sontra's inability to license the Unlicensed
Technology is a result of action taken or failed to be taken by Consultant or,
as provided in Section 6(c), the proposed activity falls inside the Consulting
Field

                                      -5-

<PAGE>

then, at Consultant's election, such dispute shall be settled by binding
arbitration in Cambridge, Massachusetts. In the arbitration proceeding,
Consultant shall select one arbitrator, Sontra shall select one arbitrator, and
the two arbitrators so selected shall select a third arbitrator. The decision of
a majority of the arbitrators shall be binding on Consultant and Sontra. Should
one party fail to select an arbitrator within five days after notice of the
appointment of an arbitrator by the other party, or should the two arbitrators
selected by Consultant and Sontra fail to select an arbitrator within ten days
after the date of the appointment of the last of such two arbitrators, any
person sitting as a Judge of the United States District Court for the Federal
District of Massachusetts in which the City of Cambridge is then situated, upon
application of Consultant or Sontra, shall appoint an arbitrator to fill such
space with the same force and effect as though such arbitrator had been
appointed in accordance with the second preceding sentence. Any arbitration
proceeding pursuant to this Section 6(a) shall be conducted in accordance with
the rules of the American Arbitration Association. Judgment may be entered on
the arbitrators' award in any court having jurisdiction.

          (e)    For a period of six months following the termination of this
Agreement, Consultant will not solicit, entice, persuade or induce any
individual who is then, or has been within the six-month period preceding
termination of this Agreement, an employee or consultant of Sontra or any of its
affiliates to terminate his or her employment with Sontra or any of its
affiliates or to become employed by or enter into contractual relations with any
other individual or entity, and the Consultant shall not approach any such
employee or consultant for any such purpose or authorize or knowingly approve
the taking of any such actions by any other individual or entity. The term
"affiliate" shall mean any person or entity that directly, or indirectly through
one or more intermediaries, controls or is controlled by, or is under common
control with, Sontra.

                                      -6-

<PAGE>

          (f)    Because a breach of the provisions of this Section 6 could not
adequately be compensated by money damages, Sontra shall be entitled, in the
event of an actual breach of this Agreement, in addition to any other right and
remedy available to it, to an injunction restraining such actual breach.
Consultant agrees that the provisions of this Section 6 are necessary and
reasonable to protect Sontra in the conduct of its business. If any restriction
contained in this Section 6 shall be deemed to be invalid, illegal, or
unenforceable by reason of the extent, duration, or geographical scope thereof,
or otherwise, then the court making such determination shall have the right to
reduce such extent, duration, geographical scope, or other provisions hereof,
and in its reduced form such restriction shall then be enforceable in the manner
contemplated hereby. Any restrictions on Consultant contained in this Section 6
shall not apply if this Agreement is terminated pursuant to Section 9 as a
result of a breach by Sontra of a material term of the Agreement.

          7.     Patents, etc.

          Sontra shall own all rights and title to any interest in patents,
patent applications, inventions, technological innovations, copyrights,
copyrightable works, developments, discoveries, designs, processes, formulas,
know-how, data and analysis, whether patentable or not, which Consultant
conceives or develops during the time in which he is rendering consulting
services to Sontra under this Agreement both (a) in connection with Consultant's
services pursuant to Section 2 herein, utilizing the time, material, facilities,
or information of Sontra, and (b) relating to a field in which Sontra may (i) be
engaged or (ii) contemplates (as demonstrated by the records of Sontra) being
engaged or conceived of or developed ("Inventions"), As soon as Consultant owns,
conceives of, or develops any Invention as described herein, he agrees
immediately to communicate such fact in writing to Sontra, and without further
compensation, forthwith upon request of Sontra, Consultant

                                      -7-

<PAGE>

shall execute all such assignments and other documents (including applications
for patents, copyrights, trademarks, and assignments thereof) and take all such
other action as Sontra may reasonably request in order (a) to vest in Sontra all
Consultant's rights, title, and interest in and to Inventions, free and clear of
liens, mortgages, security interests, pledges, charges, and encumbrances arising
from the acts of Consultant ("Liens") (Consultant to take such actions, at his
expense, as are necessary to remove all such Liens), and (b) if patentable or
copyrightable, to obtain patents or copyrights (including extensions and
renewals) thereof at Sontra's expense in any and all countries in such name as
Sontra shall determine.

          8.     Confidential Information

          (a)    All confidential or proprietary information concerning the
conduct, affairs, products, Inventions, plans, employees or other aspects of
Sontra's business, prospects or assets that Consultant may obtain from Sontra
during or after the Consulting Period, or may create prior to the end of the
Consulting Period or otherwise relating to the business of Sontra or of any
customer or supplier of Sontra shall not be published, disclosed, or made
accessible by him to any other person, firm, or corporation either during or
after the Consulting Period or used by him, either directly or indirectly,
except during the Consulting Period in the business and for the benefit of
Sontra, in each case without prior written permission of Sontra. Consultant
shall return or destroy all physical evidence of such confidential information,
to Sontra prior to or at the termination of his retention as a consultant by
Sontra. As used in this Section 8, "confidential information" shall mean any
information developed by or on behalf of, or otherwise acquired by, Sontra and
regarded by Sontra as confidential, except that information which is generally
known and available to the public, Nothing contained herein shall in any way
prohibit Consultant from publishing his research pursuant

                                      -8-

<PAGE>

to his ongoing obligations to M.I.T. subject (as currently permitted by M.I.T.)
to a reasonable period of delay to permit Sontra to perfect any patent rights
relative to such research, such delay not to exceed 90 days.

          (b)    Consultant shall use his reasonable efforts to cause all
assistants, laboratory personnel, consultants and others who may provide
services for or on behalf of any of Sontra's projects, to execute
confidentiality agreements.

          (c)    The rights of Sontra, and the obligations of the Consultant
(including, without limitation, Consultant's services set forth in Section 2)
contained in this Agreement, including, without limitation, any interest in
Inventions, are subject to and limited by the rights of M.I.T., growing out of
such entity's relationship (direct or indirect) with Consultant. In particular
with respect to paragraph 6(a), Consultant shall be free to perform services to
the extent M.I.T. reasonably deems it necessary to the exploitation of whatever
rights they may have in Inventions.

          9.     Termination

          Notwithstanding anything herein contained, if on or after the
Effective Date and prior to the end of the Consulting Period:

          (a)    either (i) Consultant shall be physically or mentally
incapacitated or disabled or otherwise unable fully to discharge his duties
hereunder for a period of six months, (ii) Sontra shall in its discretion, have
found "justifiable cause", or (iii) either party to this Agreement shall breach
any material term of this Agreement and fail to correct such breach within 20
days after notice by the other party of commission of the same, then, and in
each such case, either party shall have the right to give notice of a date (not
earlier than 20 days from such notice) to be specified in such notice, and this
Agreement shall terminate on the date so specified. For purposes of this

                                      -9-

<PAGE>

subparagraph, "justifiable cause" shall mean the willful breach or habitual
neglect of his duties hereunder.

          (b)    Consultant shall die, then this Agreement shall terminate on
the date of Consultant's death.

          (c)    Sontra shall dissolve and not be reconstituted within six
months thereafter, then this Agreement shall terminate effective on the date of
dissolution.

          (d)    M.I.T. or any other educational or health-care institution with
which Consultant is currently affiliated shall adopt conflict-of-interest or
other policies that shall be inconsistent in any material respect with the terms
and provisions of this Agreement, and Consultant and Sontra shall fail to amend
this Agreement so as to remove such conflict within 60 days following written
notification by Consultant to Sontra of the existence of such conflict (each
party hereby agreeing to negotiate in good faith during such 60-day period),
then this Agreement shall terminate.

          (e)    After the fourth anniversary of the Effective Date, either
party may terminate this Agreement upon ninety (90) days written notice to the
other party

          10.    Survival

          The covenants, agreements, representations, and warranties contained
in or made pursuant to this Agreement shall survive termination of Consultant's
services as provided herein.

          11.    Entire Agreement: Modification

          This Agreement sets forth the entire understanding of the parties with
respect to the subject matter hereof, supersedes all existing agreements between
them concerning such subject matter, and may be modified only by a written
instrument duly executed by each party.

          12.    Notices

                                      -10-

<PAGE>

Any notice or other communication required or permitted to be given hereunder
shall be in writing and shall be mailed, return receipt requested, or delivered
against receipt to the party to whom it is to be given at the address of such
party set forth in the preamble to this Agreement (or to such other address as
the party shall have furnished in writing in accordance with the provisions of
this Section 12). Notice to the estate of Consultant shall be sufficient if
addressed to Consultant as provided in this Section 12. Any notice or other
communication given by certified mail shall be deemed given three days after the
time of certification thereof, except for a notice changing a party's address
which shall be deemed given at the time of receipt thereof.

          13.    Waiver

          Any waiver by either party of a breach of any provision of this
Agreement shall not operate as or be construed to be a waiver of any other
breach of such provision or of any breach of any other provision of this
Agreement. The failure of a party to insist upon strict adherence to any term of
this Agreement on one or more occasions shall not be considered a waiver or
deprive that party of the right thereafter to insist upon strict adherence to
that term or any other term of this Agreement. Any waiver must be in writing,
signed by the party giving such waiver.

          14.    Binding Effect

          Consultant's services, rights and obligations under this Agreement
shall not be transferable by assignments or otherwise. The provisions of this
Agreement shall be binding upon and inure to the benefit of Consultant and his
heirs and personal representatives, and shall be binding upon and inure to the
benefit of Sontra and its successors and assigns. The term successors and
assigns shall include any company, partnership, association or other entity that
buys all or substantially all of Sontra's assets or stock or with which it
merges or consolidates.

                                      -11-

<PAGE>

          15.    Headings

          The headings in this Agreement are solely for the convenience of
reference and shall be given no effect in the construction or interpretation of
this Agreement.

          16.    Counterparts: Governing Law

          This Agreement may be executed in any number of counterparts, each of
which shall be deemed an original, but all of which together shall constitute
one and the same instrument. It shall be governed by and construed in accordance
with the laws of the State of Delaware, without giving effect to the conflict of
laws. Notwithstanding the above, any actions between Consultant and Sontra must
be brought either in Massachusetts State Court or the Federal Courts for the
District of Massachusetts.

          IN WITNESS WHEREOF, the parties have duly executed this Agreement on
June 1, 1998, effective as of the date first above written.

                                               SONTRA MEDICAL, INC.

                                               By: /s/ James R. McNab, Jr.
                                                  ------------------------------
                                                    James R. McNab, Jr.

                                               ROBERT S. LANGER

                                                /s/ Robert S. Langer
                                               ---------------------------------

                                      -12-

<PAGE>

                                                                      APPENDIX A
                                                                       (Langer)

                 None.

                                      -13-

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