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                                                                  EXHIBIT 10.10

                              EMPLOYMENT AGREEMENT

                  THIS EMPLOYMENT AGREEMENT (the "Agreement") is made this 28th
day of July, 2003 between Idenix Pharmaceuticals, Inc., a corporation
domesticated under the laws of the State of Delaware (together with its
successors and assigns, the "Company") and Guy Macdonald (the "Employee").

                  WHEREAS, the Company desires to employ the Employee as the
Company's Executive Vice President - Operations;

                  WHEREAS, the Employee is willing to accept such employment
with the Company; and

                  WHEREAS, the Company and the Employee desire to enter into an
employment agreement upon the terms and conditions set forth herein.

                  NOW, THEREFORE, in consideration of the mutual covenants and
representations contained herein, the Company and the Employee (individually, a
"Party" and, together, the "Parties") hereto agree as follows:

1.    EMPLOYMENT PERIOD

                  Commencing on September 2, 2003 (the "Effective Date"), the
Company shall employ the Employee, and the Employee shall serve the Company,
under the terms of this Agreement for a term (the "Term") of three (3) years
from the Effective Date (the "Initial Term"), provided, however, that the Term
shall automatically renew for successive one-year terms, unless either Party
gives written notice to the other party not less than 90 calendar days prior to
the expiration of any such term that such Party is electing not to so extend the
Term. Notwithstanding the foregoing, the Employee's employment hereunder may be
terminated at any time, subject to Section 4 and Section 5 hereof, following the
Effective Date. The period of time during which the Employee is employed
hereunder shall be referred to herein as the "Employment Period."

2.    DUTIES AND STATUS; PLACE OF EMPLOYMENT

                  During the Employment Period, the Company hereby engages the
Employee as the Company's Executive Vice President - Operations on the terms and
conditions set forth in this Agreement. During the Employment Period, the
Employee shall report to the Company's Chief Executive Officer, and shall
exercise such authority and perform such duties and functions and discharge such
responsibilities as are customarily associated with the Employee's position and
shall not be assigned duties that are materially inconsistent with his ability
to discharge the foregoing duties and responsibilities. During the Employment
Period, the Employee agrees to devote

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substantially all of the Employee's business time, efforts and skills to the
performance of the Employee's duties and responsibilities under this Agreement,
provided, however, that the Employee shall be permitted to manage his personal
investments and affairs, to serve with the prior consent of the Company's Chief
Executive Officer on such scientific advisory boards or other similar
organizations as agreed to promote the interests of the Company and to engage in
charitable activities and community affairs provided such activities do not
materially interfere with the proper performance of the Employee's duties and
responsibilities hereunder. During the Employment Period, the Employee's
principal place of employment shall be in the Boston, Massachusetts metropolitan
area.

3.    COMPENSATION AND BENEFITS

         A. Salary. During the Employment Period, the Company shall pay to the
Employee, as compensation for the performance of the Employee's duties and
obligations under this Agreement, an annual base salary of $290,000 ("Base
Salary"), payable in accordance with the normal payroll practices of the
Company. The Base Salary shall be reviewed annually for additional increases, if
any, which in the sole discretion of the Board of Directors of the Company (the
"Board") are merited or necessary to adjust for cost of living increases or to
retain a competitive Base Salary for the Employee. After any such increase, the
term "Base Salary" as utilized in this Agreement shall thereafter refer to the
increased amount. Base Salary shall not be reduced at any time without the
express prior written consent of the Employee.

         B. Annual Bonus. The bonus payable to the Employee for the period
ending December 31, 2003 shall be $100,000. During the remainder of the
Employment Period, the Employee shall be eligible for an annual target bonus
equal to no less than 30% of the Employee's Base Salary (the "Target Bonus"),
based upon criteria which, the Board, shall establish annually in consultation
with Company's senior management, which criteria shall be communicated to the
Employee in writing in a timely fashion. The Target Bonus shall be payable in
such amount if the relevant criteria are met. If such performance goals are
exceeded, the Employee shall receive a larger amount of up to 200% of the annual
target bonus opportunity. Any annual bonus shall be payable in accordance with
the terms of the bonus program established by the Board, provided that any bonus
earned shall be paid no later than 90 calendar days after the end of the
applicable performance period. The Employee's target bonus opportunity as a
percentage of Base Salary may be reviewed periodically for any increase in the
discretion of the Board. After any such increase, the term "Target Bonus" as
utilized in this Agreement shall thereafter refer to the increased amount. The
Target Bonus shall not be reduced at any time without the express prior written
consent of the Employee.

         C. Sign-on Bonus. As a further inducement to accept employment with the
Company, the Company shall pay to the Employee the amount of $200,000 (the
"Sign-

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On Bonus") not later than 90 calendar days after the commencement of the
Employment Period. Except in the case of termination by the Employee for Good
Reason, death or Disability, or by the Employer other than for Cause if the
Employment Period is terminated on or prior to the 2nd anniversary of the
Employment Period commencement, the Employee shall be liable for repayment of a
prorata portion of the Sign-on Bonus. The prorata amount repayable will be equal
to the aggregate Sign-on Bonus ($200,000) less the aggregate monthly reductions
(in the amount of $8,333.33 per month) effected prior to the termination of the
Employment Period. The monthly reductions will be effected on the last day of
each month during the 1st and 2nd year of the Employment Period. Any amounts the
Employee is obligated to repay under this Section 3.C. will be due and payable
within 90 days of the termination of the Employment Period.

         D. Equity.

                  (i)      Equity Award - Commencement of Employment Period. In
connection with the commencement of the Employment Period, the Employee will be
awarded, pursuant to the Company's 1998 Equity Incentive Plan, as amended (as
the same may be further amended, modified or supplemented, the "1998 Plan") an
option (the "Employment Commencement Option") to purchase 175,000 shares of the
Company's common stock, par value $.001 per share (the "Common Stock"). Such
award will vest ratably on a monthly basis over the 48 months subsequent to the
date of grant.

                  (ii)     Vesting. Any unvested or restricted Equity Awards
will continue to vest (or have restrictions lapse) in accordance with the terms
of this Agreement or the applicable plan and award agreement, including any
provision providing for acceleration of vesting (or lapsing of restrictions)
upon any "change in control" of the Company (as defined in the applicable plan
or award agreement) that may occur following the Effective Date. For purposes
hereof, "Equity Awards" shall mean any options to purchase Common Stock or other
shares of the Company's capital stock, any restricted shares of Common Stock or
other shares of the Company's capital stock or any other equity-based awards of
the Company granted to the Employee.

                  (iii)    Additional Equity Awards. The Employee shall be
eligible to participate in any long-term incentive award program of the Company,
including, but not limited to, the 1998 Plan and any successor thereto. In
addition, commencing with the year ending December 31, 2003, the Employee shall
have an annual performance target equity opportunity to be awarded an option to
purchase 30,000 shares of the Common Stock (with such award vesting ratably over
the 48 months following the date of grant). The partial year of service the
Employee will render to the Company in 2003 will not affect, diminish, reduce or
result in any pro-ration of such annual performance target opportunity for the
year ending December 31, 2003. The actual number of shares

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to be awarded under such target equity opportunity shall be subject to annual
approval by the Company's Board of Directors and conditioned upon the
achievement of annual performance targets established by the Board.

         E. Employee Benefits. During the Employment Period, the Employee shall
be entitled to participate in all of the employee and fringe benefit plans of
the Company in effect during the Employment Period on the same basis as provided
generally to other senior-level executives of the Company, subject to, and on a
basis consistent with, the participation requirements and other terms and
conditions of such plans. The Employee shall be entitled to four (4) weeks
vacation per calendar year, subject to such carry-over policy as is approved by
the Board. During the Employment Period, the Employee shall also be entitled to
Company paid term life insurance, which will, when aggregated with death benefit
amounts generally available to other senior level executives, provide the
Employee with a death benefit in the aggregate amount of $1 million provided
that the Employee shall be responsible for the amount of premiums on such
supplemental insurance to the extent such premiums exceed standard commercial
premiums for an individual in good health of the same age as the Employee. Upon
termination of the Employment Period for any reason other than death, the
Employee shall have the option to purchase the life insurance policy for (1) the
interpolated terminal reserve (in the event of a term policy), or (2) the cash
surrender value (in the event of a non-term policy). Within thirty (30) days of
termination of the Employment Period, the Company shall notify the Employee in
writing of his option to purchase the insurance policy, which notice shall
contain the purchase price of the policy as well as any forms required by the
insurance company to effectuate the transfer. Within thirty (30) days of receipt
of said notice, the Employee may purchase the policy by forwarding a check for
the purchase price and any forms required by the insurance company to the
Company, and the insurance policy shall be transferred to the Employee as soon
as possible. In the event the Employee does not exercise the option granted
herein, the Company shall immediately terminate the life insurance policy.

         F. Business Expenses. During the Employment Period, the Company shall
promptly reimburse the Employee for all appropriately documented, reasonable
business expenses incurred by the Employee in the performance of the Employee's
duties under this Agreement, in accordance with the Company's policies.

         G. Relocation. In connection with the Employee's relocation to the
Boston, Massachusetts metropolitan area, Idenix will reimburse the Employee for
reasonable moving and relocation expenses (not to exceed $25,000), real estate
commissions (not to exceed 6%) and conveyancing fees inclusive of Pennsylvania
Realty Transfer Taxes incurred on the sale of the Employee's current principal
residence, actual expenses of two househunting trips to Cambridge for himself
and his family, temporary housing for 90 days during the Initial Term (to be
utilized in the Employee's discretion), and amounts required to gross up the
aggregate of all of the foregoing to offset any related

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additional income tax liabilities. The Company's obligation to make the payments
contemplated by this Section 3.G will terminate on the third anniversary of the
Employment Period commencement.

4.    TERMINATION OF EMPLOYMENT

         A. Termination by the Company. The Company shall have the right to
terminate the Employment Period and the Employee's employment hereunder for any
reason at any time, including for any reason that does not constitute Cause,
subject to the consequences of such termination as provided in this Agreement.
For purposes of this Agreement, and subject to the Employee's opportunity to
cure as provided below in Section 4.C., the Company shall have "Cause" to
terminate the Employee's employment hereunder if such termination shall be the
result of: (i) willful fraud or willful material dishonesty in connection with
the Employee's employment by the Company; (ii) intentional failure by the
Employee to substantially perform the Employee's duties hereunder or gross
neglect in the performance of such duties; (iii) gross misconduct by the
Employee that is materially detrimental to the Company's reputation, goodwill or
business operations; (iv) a breach of any of the Employee's covenants as
provided in Section 7 hereof; (v) breach of any representation or warranty made
by the Employee in Section 7 hereof; (vi) the violation of any law, rule or
regulation or an indictment on a charge of commission of a felony; or (viii) the
Employee becoming subject to a civil, regulatory or criminal proceeding, or the
entering of any civil, regulatory or enforcement action (whether pursuant to a
formal or informal agreement or court order) relating to the Employee's service
as a director, officer, employee of any entity, including, but not limited to
the Company.

         B. Termination by the Employee. The Employee shall have the right to
terminate the Employment Period for any reason at any time, including for any
reason that does not constitute Good Reason, subject to the consequences of such
termination and the Employee's covenants as provided in this Agreement and such
termination shall not be deemed to be a breach of this Agreement. For purposes
of this Agreement, and subject to the Company's opportunity to cure as provided
below, the Employee shall have "Good Reason" to terminate the Employee's
employment hereunder if such termination shall be the result of the following,
without the Employee's prior written consent, provided that the basis for Good
Reason does not result from the Company taking steps to terminate the Employment
Period and the Employee's employment hereunder for Cause pursuant to Section
4.A. hereof: (i) any adverse change in the Employee's title, any change
resulting in the Employee reporting to any person other than the Company's Chief
Executive Officer or a material diminution in the Employee's authority or
responsibilities as set forth in Section 2 hereof (unless required by law,
regulation, court order or formal or informal agreement), (ii) a reduction in
the Employee's Base Salary, the Target Bonus or target equity amount, (iii) a
relocation of the Employee's primary place of employment to a location more than
40 miles from the

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Employee's current primary place of employment, or (iv) any other material
breach by the Company of this Agreement (unless such breach is mandated by law,
regulation, court order or formal or informal agreement), which breach adversely
affects the Employee and is not promptly cured after the Employee's delivery of
a written notice of such breach to the Company.

         C. Notice and Opportunity to Cure. Notwithstanding the foregoing, it
shall be a condition precedent to the Company's right to terminate the
Employee's employment for Cause pursuant to clauses (i) through (iv) of Section
4.A. above and the Employee's right to terminate the Employee's employment for
Good Reason that (1) the party seeking the termination shall first have given
the other party written notice stating with reasonable specificity the reason
for the termination ("breach"), (2) if such breach is susceptible of cure or
remedy, a period of 30 calendar days from and after the giving of such notice
shall have elapsed without the breaching party having effectively cured or
remedied such breach during such 30-day period and (3) in the case of a
termination for Cause, if the Employee fails to cure such neglect or conduct
within such 30-day period, the Employee has an opportunity to be heard before
the Board or, if the Board shall so designate, before a committee comprised of
the Chief Executive Officer of the Company and two other senior executives of
the Company, in each such case subject to a majority vote to terminate the
Employee's employment for Cause.

         D. Termination Upon Death or Disability. The Employment Period and the
Employee's employment hereunder shall be terminated by the death of the
Employee. The Employment Period and the Employee's employment hereunder may be
terminated by the Company or the Employee if the Employee shall be rendered
incapable of performing the Employee's duties to the Company by reason of any
medically determined physical or mental impairment that can reasonably be
expected to result in death or has lasted for a period of six (6) or more
consecutive months from the first date of the Employee's absence due to the
disability (a "Disability").

         E. Expiration of Term. The Employment Period shall terminate upon the
expiration of the Term. Any continuation of the Employee's employment beyond the
expiration of the original Term or any extended Term shall not be subject to the
terms and conditions of this Agreement, except that the "Inventions Agreement"
set forth as Exhibit A shall survive the Term and the expiration of the
Employment Period, and Sections 7 and 9 hereof shall survive in accordance with
their terms for the periods specified therein.

5.    CONSEQUENCES OF TERMINATION

         A. Without Cause; For Good Reason. In the event of a termination of the
Employee's employment during the Employment Period by the Company other than for

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Cause or by the Employee for Good Reason (a "Covered Termination"), the Company
shall provide the Employee with the following:

                  (i)      payment of a cash lump-sum amount equal to the sum of
         (x) the Employee's Base Salary and (y) the greater of (1) Target Bonus
         or (2) the bonus earned by the Employee for the year preceding the year
         in which termination of employment occurs, payable as soon as
         practicable following the termination of employment and the expiration
         of the revocation period in the Employee Release (as defined in Section
         8 hereof);

                  (ii)     immediate vesting and exercisability of all
         outstanding equity awards, including any Equity Awards, upon a Covered
         Termination, with any equity awards that are stock options remaining
         exercisable for 24 months following the date of a Covered Termination
         (or until the expiration of the term of the option, if earlier);

                  (iii)    continued coverage for the Employee and the
         Employee's eligible dependents under all group medical, dental and life
         insurance coverages that are provided to employees of the Company
         generally for a period of 12 months following a Covered Termination,
         with such coverage to be at the Company's cost (subject to standard
         employee contribution requirements). Any such coverage shall be
         discontinued in the event that the Employee obtains substitute coverage
         from subsequent employment or service during such 12-month period; and

                  (iv)     payment of (x) any earned but unpaid amounts as of
         the date of termination, including, but not limited to, Base Salary
         through the date of termination, reimbursement of business expenses and
         any incentive awards earned for performance periods that have ended,
         (y) any compensation previously deferred by the Employee together with
         any vested Company matching contributions and (z) any accrued but
         unpaid vacation days under Company policy through the date of
         termination ("Accrued Obligations"), payable as soon as practicable
         following such termination.

         B. Death or Disability. In the event the Employee's employment with the
Company is terminated on account of death or Disability during the Employment
Period, the Company shall provide the Employee (or his estate or legal
representative, as the case may be) with such death or disability benefits as
are provided under the death and disability plans that are available to
employees of the Company generally on the date of termination. In addition, the
Employee (or his estate or legal representative, as the case may be) shall be
entitled to:

                  (i)      in the case of Disability, payment of 60% of Base
         Salary per year until the Employee reaches age 65 (with such payment
         obligation offset by any

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         amounts the Employee receives from any insurance plan or policy of the
         Company);

                  (ii)     immediate vesting and exercisability of all
         outstanding equity awards, including any Equity Awards, upon such
         termination of employment, with any equity awards that are stock
         options remaining exercisable for 24 months following the date of
         termination (or until the expiration of the term of the option, if
         earlier);

                  (iii)    continued coverage for the Employee (in the case of
         Disability) and the Employee's eligible dependents under all group
         medical, dental and life insurance coverages that are provided to
         employees of the Company generally for a period of 12 months following
         such termination, with such coverage to be at the Company's cost
         (subject to standard employee contribution requirements). Any such
         coverage shall be discontinued in the event that the Employee obtains
         substitute coverage from subsequent employment or service during such
         12-month period; and

                  (iv)     payment of Accrued Obligations as soon as practicable
         following the termination of employment.

         C.    Other Termination.

                  (i)      In the event that the Employee's employment with the
Company is terminated during the Employment Period (i) by the Company for Cause,
(ii) by the Employee other than for Good Reason, or (iii) as a result of the
death or Disability of the Employee, the Employee shall not be entitled to any
further payments, compensation or other benefits under this Agreement, except
the Employee shall be entitled to (i) payment of Accrued Obligations as soon as
practicable following such termination and (ii) treatment of any outstanding
equity awards in accordance with the applicable plan or award agreement.

                  (ii)     In the event that the Company elects not to extend
the Term beyond the Initial Term, to the extent that the Employment Commencement
Option or shares of Common Stock acquired by the Employee pursuant to the
exercise thereof remains unvested or subject to restrictions intended to lapse
with the passage of time (the "Vesting Restrictions"), such option shall
immediately vest and become exercisable in full for a period of 15 months after
the expiration of the Initial Term and the Vesting Restrictions will immediately
lapse upon the expiration of the Initial Term.

         D. Other Benefits. The benefits payable to the Employee under this
Agreement are not in lieu of any benefits payable or entitlements due under any
applicable plan, program, policy or arrangement of, or other agreement with, the
Company or any

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affiliate, and upon termination the Employee shall receive such benefits,
payments or entitlements, if any, as he may be entitled to receive pursuant to
the terms of such plans, programs, policies, arrangements or other agreements.
Except for the obligations of the Company provided by the foregoing and this
Section 5, the Company shall have no further severance obligations to the
Employee or the Employee's beneficiaries upon the Employee's termination of
employment.

         E. No Mitigation or Offset. The Employee shall have no obligation to
mitigate the damages provided by this Section 5 by seeking substitute employment
or otherwise and, except as provided in Section 5.A(iii) and Section 5.B(iii)
hereof, there shall be no offset of the payments or benefits provided in this
Section 5. In addition, except as provided in Section 7.E hereof, the Company's
obligation to make any payment pursuant to, or otherwise perform its obligations
under, this Agreement shall not be affected by any claim or other right the
Company or any affiliate may have against the Employee for any reason.

6.    CHANGE IN CONTROL PROTECTIONS

         A. Termination Within One Year After a Change in Control. If the
Employee's employment is terminated by the Company without Cause or the Employee
terminates his employment for Good Reason, in each case within one (1) year
following a Change in Control and during the Term, in addition to the payments,
benefits and entitlements provided pursuant to Section 5.A hereof, the Employee
shall also be entitled to an additional lump-sum amount equal to the sum of (x)
the Employee's Base Salary and (y) the greater of (1) Target Bonus or (2) the
bonus earned by the Employee for the year preceding the year in which
termination of employment occurs, payable as soon as practicable following the
termination of employment.

         B. Excise Tax Provision. Anything herein to the contrary
notwithstanding, to the extent that any payment, entitlement or benefit provided
under this Agreement or any other agreement, plan, policy, program or
arrangement of the Company (the "Payments") would be subject to the imposition
of the excise tax imposed under Section 4999 of the Internal Revenue Code of
1986, as amended, or any similar Federal or state law (an "Excise Tax"), the
Payments shall be reduced (but not below zero) to the maximum amount as will
result in no portion of the Payments being subject to such Excise Tax (the "Safe
Harbor Cap"), but only if the net after-tax amount that would be received by the
Employee, taking into account all applicable Federal, state and local income
taxes and the imposition of the Excise Tax, is greater than the net after-tax
amount that would be received by the Employee if Payments are not reduced to the
Safe Harbor Cap. Unless the Employee has given prior written notice specifying a
different order to the Company to effectuate the reductions described in the
preceding sentence, the Company shall reduce or eliminate the Payments to the
Safe Harbor Cap, by first reducing or eliminating those payments or benefits
which are not payable in cash and

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then by reducing or eliminating cash payments. Any notice given by the Employee
pursuant to the preceding sentence shall take precedence over the provisions of
any other plan, arrangement or agreement governing the Employee's rights and
entitlements to any benefit, entitlement or compensation.

         C. Definition of Change in Control. For purposes of Section 3 hereof
and this Section 6, the term "Change in Control" shall mean:

                           (i)      any "person," as such term is used in
                  Sections 3(a)(9) and 13(d) of the Securities Exchange Act of
                  1934, becomes a "beneficial owner," as such term is used in
                  Rule 13d-3 promulgated under that act, of fifty percent (50%)
                  or more of the Voting Stock of the Company, or Novartis Pharma
                  AG disposes of its entire interest in the Company's Voting
                  Stock (except in connection with an initial public offering of
                  the Voting Stock);

                           (ii)     the Company adopts any plan of liquidation
                  providing for the distribution of all or substantially all of
                  its assets;

                           (iii)    all or substantially all of the assets or
                  business of the Company is disposed of pursuant to a merger,
                  consolidation or other transaction (unless the shareholders of
                  the Company immediately prior to such merger, consolidation or
                  other transaction beneficially own, directly or indirectly, at
                  least fifty percent (50%) of the Voting Stock or other
                  ownership interests of the entity or entities, if any, that
                  succeed to the business of the Company); or

                           (iv)     the Company combines with another company
                  and is the surviving corporation but, immediately after the
                  combination, the shareholders of the Company immediately prior
                  to the combination hold, directly or indirectly, fifty percent
                  (50%) or less of the Voting Stock of the combined company
                  (there being excluded from the number of shares held by such
                  shareholders, but not from the Voting Stock of the combined
                  company, any shares received by affiliates of such other
                  company in exchange for stock of such other company).

         For purposes of this definition of "Change in Control" the "Company"
shall include any entity that succeeds to all or substantially all of the
business of the Company and "Voting Stock" shall mean securities of any class or
classes having general voting power under ordinary circumstances, in the absence
of contingencies, to elect the directors of a corporation.

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7.    REPRESENTATIONS AND COVENANTS

         A. Representations of Employee. The Employee represents and warrants
that (i) he has the free and unfettered right to enter into this Agreement and
to perform his obligations under the term of this Agreement and that the
Employee knows of no agreement between him and any other person, firm or
organization, or any law or regulation that would be violated by the performance
of his obligations under this Agreement; (ii) he is not aware of any regulatory,
court or other proceeding relating to his service as a director, officer or
employee of any other entity; and (iii) he has not been charged with, or
indicted for, any crime.

         B. Inventions. The Employee agrees to sign and honor an "Inventions
Agreement" in the form attached hereto as Exhibit A.

         C. Nonsolicitation of Employees. During the Restricted Period (as
defined in Section 7.E below), the Employee shall not, without the express prior
written approval of the Company, (i) directly or indirectly, knowingly solicit
any employee of the Company or any of its affiliates (or any employee who was
employed by the Company or any of its affiliates at any time within six (6)
months prior to the date the Employee seeks to solicit such person ("Former
Employee")) to leave the employ of the Company or its affiliates, as the case
may be.

         D. Nonsolicitation of Customers. During the Restricted Period, the
Employee agrees that other than in the ordinary course of business he shall not,
without the express prior written approval of the Company, knowingly solicit,
recruit or encourage any customer of the Company or any of its affiliates who
was a customer at, or was a customer within the six (6)-month period preceding,
the date of the Employee's termination of employment to reduce or cease its
business with the Company or any such affiliate.

         E. Noncompetition. In consideration of the employment of the Employee
by the Company under the terms of this Agreement, the Employee shall not, during
the Restricted Period, without the express written approval of the Company,
other than in the ordinary course of performing his duties hereunder, engage in
a "Competitive Business," directly or indirectly, as an individual, partner,
shareholder, director, officer, principal, agent, employee, trustee, consultant,
or in any relationship or capacity, in any geographic location in which the
Company or any of its Affiliates is engaged in business. Anything to the
contrary notwithstanding, it shall not be a violation of this Section 7.E for
the Employee to (i) own or acquire up to two percent (2%) of the outstanding
equity securities (measured by value) of any entity, (ii) serve as a member of
the board of directors or as a member of an advisory committee of any entity on
which Employee was serving prior to the date of termination of his employment or
of any entity which is not engaged in a Competitive Business or (iii) provide
services to a

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subsidiary, division or affiliate of a Competitive Business if such subsidiary,
division or affiliate is not itself engaged in a Competitive Business and the
Employee does not provide services to, or have any responsibilities regarding,
the Competitive Business.

         For purposes of this Agreement, the term "Competitive Business" shall
mean a commercial, for profit entity that discovers, develops and commercializes
therapeutics for the treatment of human viral diseases in the same geographic
area that the therapeutics for the treatment of human viral diseases are
discovered, developed, marketed and commercialized by the Company or any of its
subsidiaries. For purposes hereof, the "Restricted Period" shall be the
Employment Period and a period of one (1) year following the termination of the
Employment Period.

         F. Enforcement. The Employee acknowledges that if he breaches any
provision of this Section 7, the Company will suffer irreparable injury. It is
therefore agreed that the Company shall have the right, if permitted by a court
of the applicable jurisdiction, to enjoin any such breach, without posting any
bond. The Employee hereby waives the adequacy of a remedy at law as a defense to
such relief. The existence of this right to injunctive, or other equitable
relief, shall not limit any other rights or remedies which the Company may have
at law or in equity including, without limitation, the right to monetary,
compensatory and punitive damages. The Employee acknowledges and agrees that the
provisions of this Section 7 are reasonable and necessary for the successful
operation of the Company. In the event a court of competent jurisdiction
determines that the Employee has breached the Employee's obligations in any
material respect under this Section 7 (other than through the issuance of an
injunction issued without a determination on the merits), the Company, in
addition to pursuing all available remedies under this Agreement, at law or
otherwise, and without limiting its right to pursue the same, shall be entitled
to cease all payments due to the Employee under this Agreement as of the date of
such determination.

8.    WAIVER

                  All payments and benefits provided by Section 5.A hereof are
conditioned upon the Employee executing and honoring a release of claims in
favor of the Company following the Employee's termination of employment in a
form attached hereto as Exhibit B (the "Employee Release").

9.    DISPUTE RESOLUTION

                  Except as otherwise provided in Section 7.E hereof, any
controversy, dispute or claim arising out of or relating to this Agreement shall
be resolved by final and binding arbitration, to be held in Boston,
Massachusetts, in accordance with the Commercial Arbitration Rules (and not the
National Rules for the Resolution of

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Employment Disputes) of the American Arbitration Association and this Section 9.
Judgment upon the award rendered by the arbitrator(s) may be entered in any
court having jurisdiction thereof. Neither Party shall be liable for punitive or
exemplary damages.

10.   WITHHOLDING OF TAXES

                  All payments required to be made by the Company to the
Employee under this Agreement shall be subject to the withholding of such
amounts relating to income tax, employment tax and such other taxes and
withholdings as the Company may reasonably determine it should withhold pursuant
to any applicable U.S. Federal, state or local law or regulation.

11.   NOTICE

                  All notices, requests and other communications pursuant to
this Agreement shall be in writing and shall be deemed to have been duly given,
if delivered in person or by courier, sent by express, registered or certified
mail, postage prepaid, or sent by facsimile transmission, addressed to the
Employee at the Employee's personal residence as reflected in the Company's
records, and to the Company at 125 CambridgePark Drive, Cambridge, Massachusetts
02140. Either party may, by written notice to the other in accordance herewith,
change the address to which notices to such party are to be delivered or mailed.

12.   GOVERNING LAW

                  This Agreement shall be construed, interpreted and enforced in
accordance with the laws of the Commonwealth of Massachusetts, without giving
effect to the choice of law principles thereof.

13.   WAIVER OF BREACH

Any waiver of any breach of this Agreement shall not be construed to be a
continuing waiver or consent to any subsequent breach on the part either of the
Employee or of the Company. Any waiver to be effective must be in writing and
signed by the Party against whom it is being enforced (either the Employee or an
authorized officer of the Company, as the case may be) and must specifically
refer to the provision of this Agreement which is being waived.

14.   NON-ASSIGNMENT; SUCCESSORS

                  This Agreement shall be binding upon and inure to the benefit
of the Parties and their respective successors, heirs (in the case of the
Employee) and assigns. No rights or obligations of the Company under this
Agreement may be assigned or

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<PAGE>

transferred by the Company except that such rights or obligations may be
assigned or transferred pursuant to a merger or consolidation in which the
Company is not the continuing entity, or the sale or other disposition of all or
substantially all of the assets of the Company, provided that the assignee or
transferee is the successor to all or substantially all of the assets of the
Company and such assignee or transferee assumes the liabilities, obligations and
duties of the Company, as contained in this Agreement, either contractually or
as a matter of law. No rights or obligations of the Employee under this
Agreement may be assigned or transferred by the Employee other than his accrued
rights to compensation and benefits, which may be transferred only by will or
operation of law, except as provided in this Section 14 or in an applicable
plan, program, grant or agreement of the Company or any affiliate. In the event
of the Employee's death or a judicial determination of his incompetence,
references in this Agreement to the Employee shall be deemed, where appropriate,
to refer to his beneficiary, estate or other legal representative.

15.   SEVERABILITY

                  If any provision of this Agreement is determined by a court of
competent jurisdiction to be not enforceable in the manner set forth in this
Agreement, the Employee and the Company agree that it is the intention of the
parties that such provision should be enforceable to the maximum extent possible
under applicable law. If any provisions of this Agreement are held to be invalid
or unenforceable, such invalidation or unenforceability shall not affect the
validity or enforceability of any other provision of this Agreement (or any
portion thereof).

16.   ENTIRE AGREEMENT

                  This Agreement and the Inventions Agreement constitute the
entire agreement by the Company and the Employee with respect to the subject
matter hereof and except as specifically provided herein, supersedes any and all
prior agreements or understandings between the Employee and the Company with
respect to the subject matter hereof, whether written or oral. Any award
agreement relating to an Equity Award, to the extent inconsistent with any
provision of this Agreement, shall be treated as amended in a manner consistent
with the terms of this Agreement. This Agreement may be amended or modified only
by a written instrument (specifically referencing the provision of this
Agreement being so amended) executed by the Employee and an authorized officer
of the Company.

17.   COUNTERPARTS

                  This Agreement may be executed in two or more counterparts.

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                  IN WITNESS WHEREOF, the parties have executed this Agreement
as of the date first written above and this Agreement shall be effective and
binding on the Parties from such date.

                                              IDENIX PHARMACEUTICALS, INC.

                                                  /s/ Jean-Pierre Sommadossi
                                              ----------------------------------
                                              By:  Jean-Pierre Sommadossi, Ph.D.
                                              Its: Chief Executive Officer

                                              EMPLOYEE

                                                  /s/ Guy Macdonald
                                              ----------------------------------
                                              Guy Macdonald

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