<PAGE>   1

                                                                   EXHIBIT 10.31


                              EMPLOYMENT AGREEMENT



         THIS EMPLOYMENT AGREEMENT (the "Agreement"), made as of April 3, 1997
is entered into by OraVax, Inc., a Delaware corporation with its principal place
of business at 38 Sydney Street, Cambridge, Massachusetts 02139 (the "Company"),
and Robert J. Gerety, residing at 103 Livingston Road, Wellesley, MA 02181 (the
"Employee").

         The Company desires to employ the Employee, and the Employee desires to
be employed by the Company. In consideration of the mutual covenants and
promises contained herein, and other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged by the parties hereto,
the parties agree as follows:

         1.    TERM OF EMPLOYMENT. The Company hereby agrees to employ the
Employee, and the Employee hereby accepts employment with the Company, upon the
terms set forth in this Agreement, for the period commencing on April 3, 1997
(the "Commencement Date") and ending on April 3, 2000 (such period, as it may be
extended, the "Employment Period"), unless sooner terminated in accordance with
the provisions of Section 4. The Employment Period will be automatically
extended, after the initial three-year term, for successive twelve-month periods
unless terminated by either party by written notice to the other not less than
six months prior to the commencement of the next succeeding twelve-month term.

         2.    TITLE; CAPACITY. The Employee shall serve as Vice President of
Development and Regulatory Affairs or in such other position as the Company's
President may determine from time to time. The Employee shall initially be based
at the Company's offices in Cambridge, Massachusetts. The Employee shall be
subject to the supervision of, and shall have such authority as is delegated to
him by, the President of the Company.

         The Employee hereby accepts such employment and agrees to undertake the
duties and responsibilities inherent in such position and such other duties and
responsibilities as the President of the Company shall from time to time
reasonably assign to him. The Employee agrees to devote his entire business
time, attention and energies to the business and interests of the Company during
the Employment Period; provided, however, that the Employee may engage in
consulting activities as specified in Schedule C hereto. The Employee agrees to
abide by the rules, regulations, instructions, personnel practices and policies
of the Company and any changes therein which may be adopted from time to time by
the Company.

         3.   COMPENSATION AND BENEFITS.

              a.   SALARY; BONUS. The Company shall pay the Employee, in monthly
installments, a base salary at the annual rate of $250,000 for the balance of
the 1997 calendar year, commencing on the Commencement Date. Such salary shall
be subject to adjustment thereafter as determined by the President and the
Compensation Committee of the Board of Directors (the "Compensation Committee").

              In addition, the Employee shall participate annually in the bonus
program for executive officers of the Corporation, which is administered by the
Compensation Committee. The Employee will be eligible for a bonus of up to 20%
of base salary for the balance of the 1997 calendar year after the Commencement
Date.





                                       1

<PAGE>   2

              b.   FRINGE BENEFITS. The Employee shall be entitled to
participate in all bonus and benefit programs that the Company establishes and
makes available to its employees to the extent that Employee's position, tenure,
salary, age, health and other qualifications make him eligible to participate,
including, but not limited to, the health, dental and disability programs
indicated on SCHEDULE A to this Agreement. In addition, the Company shall assume
payment during the Employment Period of the premiums on existing individual term
life insurance policies on the Employee's life with Equitable, Prudential,
Serviceman's Group Life and Sun Life, designating family member(s) of his choice
as beneficiary(ies), with combined coverage in the face amount of approximately
$1,050,000, each such payment to be made by the Company promptly upon submission
of the bills by the Employee.

              c.   VACATION. The Employee shall be entitled to three weeks paid
vacation per year, to be taken at such times as may be approved by the President
of the Company. The three weeks of paid vacation shall accrue as follows: (i) in
year one, (a) the Employee shall receive one day of vacation per month worked
for the first six months, and (b) after the first six months of employment have
passed, the Employee shall receive the remaining number of vacation days
necessary to bring the annual total of vacation days accrued to three full weeks
of vacation; and (ii) after year one, the three weeks of vacation shall accrue
on the anniversary of the Commencement Date.

              d.   REIMBURSEMENT OF EXPENSES. The Company shall reimburse the
Employee for all reasonable travel, entertainment and other expenses incurred or
paid by the Employee in connection with, or related to, the performance of his
duties, responsibilities or services under this Agreement, upon presentation by
the Employee of documentation, expense statements, vouchers and/or such other
supporting information as the Company may request, PROVIDED, HOWEVER, that the
amount available for such travel, entertainment and other expenses may be fixed
in advance by the Board.

              e.   STOCK OPTIONS. As approved by the Company's Board of
Directors (the "Board"), the Company grants to the Employee an incentive stock
option to purchase 125,000 shares of the Company's Common Stock, $.001 par value
per share (the "Common Stock") at fair market value on the Commencement Date.
The stock option agreement evidencing such option shall provide that the option
shall become exercisable as to installments of 31,250 shares on each of the
first four anniversary dates of the date of grant. In the event of a Change in
Control of the Company as defined in the stock option agreement, such option
shall become fully exercisable.

         4.   EMPLOYMENT TERMINATION. The employment of the Employee by the
Company pursuant to this Agreement shall terminate upon the occurrence of any of
the following:

              a.   Expiration of the Employment Period in accordance with
Section 1;

              b.   At the election of the Company, for cause, immediately upon
written notice by the Company to the Employee. For the purposes of this Section
4.2, cause for termination shall be deemed to exist only in the event of (a) the
willful and continued failure by the Employee to substantially perform his
duties hereunder (other than any such failure resulting from the Employee's
inability to perform such duties as a result of physical or mental illness or
incapacity) after delivery to the Employee of a written demand for substantial
performance that specifically identifies the manner in which the Company
believes that the Employee has not substantially performed his duties and
provides a reasonable opportunity to cure; (b) dishonesty, gross negligence or
willful misconduct; or (c) the conviction of the Employee of, or the entry of a
pleading of guilty or nolo contendere by the Employee to, any crime involving
moral turpitude or any felony. No act or failure to act shall be considered


                                       2
<PAGE>   3

"willful" for this purpose unless done, or omitted to be done, by the Employee
other than in good faith and other than with a reasonable belief that his action
or omission was in the best interests of the Company. The Employee shall not be
deemed to have been terminated for Cause unless the Employee shall have been
provided with (i) a reasonable notice, setting forth the reasons that the
Company believes constitute Cause for the termination of his employment; (ii) a
reasonable opportunity to be heard by the Board, with his counsel; and (iii) a
notice of termination from the Board finding that, in the reasonable good faith
opinion of the Board, Cause for the termination exists and setting forth in
reasonable detail the facts and circumstances relied upon as providing a basis
for the termination under the provision so specified.

              c.   Thirty days after the death or disability of the Employee. As
used in this Agreement, the term "disability" shall mean the inability of the
Employee, due to a physical or mental disability, for a period of 90 consecutive
days during any 365-day period to perform the services contemplated under this
Agreement on a substantially full-time basis. A determination of disability
shall be made by a physician satisfactory to both the Employee and the Company,
PROVIDED THAT if the Employee and the Company do not agree on a physician, the
Employee and the Company shall each select a physician and these two together
shall select a third physician, whose determination as to disability shall be
binding on all parties;

              d.   At the election of the Company, upon written notice of
termination;or

              e.   At the election of the Employee, upon written notice to the
Company of not less than six months.

     5.  EFFECT OF TERMINATION.

              a.   TERMINATION FOR CAUSE OR AT ELECTION OF EITHER PARTY. In the
event the Employee's employment is terminated for cause pursuant to Section 4.2
or at the election of the Employee pursuant to Section 4.5, the Company shall
pay to the Employee the compensation and benefits otherwise payable to him under
Section 3 through the last day of his actual employment by the Company.

              b.   TERMINATION FOR DEATH OR DISABILITY. If the Employee's
employment is terminated by death or because of disability pursuant to Section
4.3, the Company shall pay to the estate of the Employee or to the Employee, as
the case may be, the compensation which would otherwise be payable to the
Employee up to the end of the month in which the termination of his employment
because of death or disability occurs.

              c.   0TERMINATION WITHOUT CAUSE. In the event the Employee's
employment is terminated without cause pursuant to Section 4.4, the Company
shall pay to the Employee compensation and benefits for a period of six months
from the last day of the Employee's actual employment by the Company.

              d.   SURVIVAL. The provisions of Sections 6 and 7 shall survive
the termination of this Agreement.


                                       3
<PAGE>   4

         6.   NON-COMPETE.

              i.     During the Employment Period and for a period of one year
after the termination or expiration thereof, the Employee will not directly or
indirectly:

                     (1)   as an individual proprietor, partner, stockholder,
officer, employee, director, joint venturer, investor, lender, or in any other
capacity whatsoever (other than as the holder of not more than one percent (1%)
of the total outstanding stock of a publicly held company), engage in the
business of, or accept employment with a business which is engaged in the
business of, developing, producing, marketing or selling products of the kind or
type developed or being developed, produced, marketed or sold by the Company
while the Employee was employed by the Company; and the Employee acknowledges
that the following companies are major competitors of the Company, employment
with which would be damaging to the Company and would violate this covenant:
Virus Research Institute, Inc.; Astra; and Chiron; and PROVIDED, FURTHER, that,
subject to the prior approval of the Company, which will not be unreasonably
withheld, the Employee may become an employee of a business in the vaccine
industry which does not compete with the Company; or

                     (2)   recruit, solicit or induce, or attempt to induce, any
employee or employees of the Company to terminate their employment with, or
otherwise cease their relationship with, the Company; or

                     (3)   solicit, divert or take away, or attempt to divert or
to take away, the business or patronage of any of the clients, customers or
accounts, or prospective clients, customers or accounts, of the Company which
were contacted, solicited or served by the Employee while employed by the
Company.

              ii.    If any restriction set forth in this Section 6 is found by
any court of competent jurisdiction to be unenforceable because it extends for
too long a period of time or over too great a range of activities or in too
broad a geographic area, it shall be interpreted to extend only over the maximum
period of time, range of activities or geographic area as to which it may be
enforceable.

              iii.   The restrictions contained in this Section 6 are necessary
for the protection of the business and goodwill of the Company and are
considered by the Employee to be reasonable for such purpose. The Employee
agrees that any breach of this Section 6 will cause the Company substantial and
irrevocable damage and therefore, in the event of any such breach, in addition
to such other remedies which may be available, the Company shall have the right
to seek specific performance and injunctive relief.

              iv.    The Employee's performance of consulting services in
conformity with the terms and conditions of Schedule C to this Agreement shall
not be deemed to violate the provisions of this Section 6.

         7.   Proprietary Information And Developments.

              a.     PROPRIETARY INFORMATION AND DEVELOPMENTS. Concurrently,
with the execution of this Agreement, the Employee shall execute and deliver an
Employee Confidential Information and Invention Assignment Agreement in the form
attached hereto as SCHEDULE B.

              b.     OTHER AGREEMENTS. Employee hereby represents that he is not
bound by the terms of any agreement with any previous employer or other party to
refrain from using or disclosing any trade secret or confidential or proprietary
information in the course of his


                                       4
<PAGE>   5

employment with the Company or to refrain from competing directly or indirectly,
with the business of such previous employer or any other party, except for an
Invention and Nondisclosure Agreement with Immulogic Pharmaceuticals Corporation
dated October 16, 1993 and a proprietary information and inventions agreement
with Aviron. Employee further represents that his performance of all the terms
of the terms of this Agreement and as an employee of the Company does not and
will not breach any agreement to keep in confidence proprietary information,
knowledge or data acquired by him in confidence or in trust prior to his
employment with the Company.

         8.   NOTICES. All notices required or permitted under this Agreement
shall be in writing and shall be deemed effective upon personal delivery or upon
deposit in the United States Post Office, by registered or certified mail,
postage prepaid, addressed to the other party at the address shown above, or at
such other address or addresses as either party shall designate to the other in
accordance with this Section 8.

         9.   PRONOUNS. Whenever the context may require, any pronouns used in
this Agreement shall include the corresponding masculine, feminine or neuter
forms, and the singular forms of nouns and pronouns shall include the plural,
and vice versa.

         10.  ENTIRE AGREEMENT. This Agreement constitutes the entire agreement
between the parties and supersedes all prior agreements and understandings,
whether written or oral, relating to the subject matter of this Agreement.

         11.  AMENDMENT. This Agreement may be amended or modified only by a
written instrument executed by both the Company and the Employee.

         12.  GOVERNING LAW. This Agreement shall be construed, interpreted and
enforced in accordance with the laws of the Commonwealth of Massachusetts.

         13.  SUCCESSORS AND ASSIGNS. This Agreement shall be binding upon and
inure to the benefit of both parties and their respective successors and
assigns, including any corporation with which or into which the Company may be
merged or which may succeed to its assets or business, provided, however, that
the obligations of the Employee are personal and shall not be assigned by him.

         14.  MISCELLANEOUS.

              a.   No delay or omission by the Company in exercising any right
under this Agreement shall operate as a waiver of that or any other right. A
waiver or consent given by the Company on any one occasion shall be effective
only in that instance and shall not be construed as a bar or waiver of any right
on any other occasion.

              b.   The captions of the sections of this Agreement are for
convenience of reference only and in no way define, limit or affect the scope or
substance of any section of this Agreement.

              c.   In case any provision of this Agreement shall be invalid,
illegal or otherwise unenforceable, the validity, legality and enforceability of
the remaining provisions shall in no way be affected or impaired thereby.



                                       5
<PAGE>   6


         IN WITNESS WHEREOF, the parties hereto have executed this Agreement as
of the day and year set forth above.



                                                   ORAVAX, INC.


                                                   By: /s/ Lance K. Gordon
                                                      --------------------------
                                                   Title: President


                                                   EMPLOYEE


                                                   /s/ Robert J. Gerety
                                                   -----------------------------
                                                   Robert J. Gerety




                                       6
<PAGE>   7

                                   SCHEDULE A

                                 FRINGE BENEFITS

    The bonus and benefit programs in effect at the Company at the Commencement
Date and in which the Employee is eligible to participate are listed in this
Schedule A:

    HEALTH INSURANCE: Prudential Pru-Care Plus;  Preferred Provider Option (PPO)
    Coverage: 100/80 Network/Outside Network; Annual Deductible: $200 per
    person/$500 per Family when out-of-network services are performed.
    Enrollment: First day of the next month following date of hire. Fully Paid
    by OraVax, Inc.

    DENTAL INSURANCE: Delta Dental Plan of Massachusetts; Preferred Provider
    Option (Approximately 95 percent of all dentists in MA participate in the
    plan). Coverage: 100%, 100%, 50% Type I, Type II, Type III; Annual
    Deductible: $25 per person/$75 per family on Type II and III Three services
    only. Enrollment: First day of the next month following date of hire.
    Fully Paid by OraVax, Inc.

    TERM LIFE INSURANCE: In lieu of the customary term life insurance plan for
    executives, the Company has agreed to assume payment of the premiums on
    existing policies as set forth in Section 3.2 of the Employment Agreement.

    BONUS. The Employee shall participate annually in the Company's bonus
    program for executives. There is no written bonus plan.

    LONG TERM DISABILITY: Available upon loss of employment greater than 90 days
    due to illness or injury. Monthly Benefit: 60% of monthly earnings to a
    maximum of $10,000 per month. Enrollment: One completion of 90 days
    full-time service. Fully paid by OraVax, Inc.

    VOLUNTARY LONG TERM DISABILITY: Voluntary participation whereby through
    company "gross-up" of payroll, the premiums are treated "as if paid" by the
    employee. Employees pay only the taxes on premiums resulting in
    significantly reduced income taxes on any proceeds of the policy.

    GROUP TERM TRAVEL/ACCIDENT INSURANCE: Available to all full-time employees
    traveling on corporate business. Benefit: $100,000 per employee payable to
    the designated beneficiary. Fully Paid by OraVax, Inc.

    DEPENDENT CARE: Tax-based program designed for employees to withhold gross
    earnings on a pre-tax basis to be reimbursed for qualified child or
    dependent day care expenses, resulting in up to $5,000 of earnings being
    income tax free.

    ORAVAX 401(K) PLAN: Allows employees to set aside up to 15% of pre-tax gross
    earnings free of Federal and State Tax Withholding in the plan. Available to
    all employees after three months of service. Enrollment: July 1 and January
    1.

    OraVax, Inc. current makes a matching contribution of 50 percent of the
    first 6% of gross pay, contributed in shares of OraVax, Inc. Common Stock.

    EMPLOYEE STOCK PURCHASE PLAN: Allows employees the opportunity to purchase
    company stock through payroll deductions during two annual offering two
    annual offering periods


<PAGE>   8

    per year. The offering periods are 1 April to 30 September and 1 October to
    31 March. The contribution range for the program is 1 to 15 percent. The
    stock is purchased at a 15 percent discount from the lower price at the
    beginning of the period or at the end of the offering period. Participants
    are required to open an account with DLJ Securities Corporation in order to
    participate. Available to employee after three months of full-time
    employment. Enrollment: 1 April and 1 October.

    TUITION REIMBURSEMENT: OraVax will reimburse employees for expenses incurred
    with continuing education up to the IRS limits. The course must be
    preapproved by Company management and completed with a grade of B or better
    for consideration. 50% of tuition and books for unrelated courses. 100% of
    tuition and books for accredited courses that are directly related to an
    employee's position within the company.

    PARKING: Each full-time employee is entitled to one parking pass for parking
    in the Forest City lots. The company pays the $85 per month cost of the
    parking pass.

    SHUTTLE SERVICE: Forest City Management provides a complimentary shuttle
    service to and from Central Square daily for MBTA buses and subway service.
    This service makes regular runs during select business hours and is
    available "on-call after hours.

         The above are summaries only. In the event of discrepancy, the plan
         document will govern the program.

<PAGE>   9

                                   SCHEDULE B

                        EMPLOYEE CONFIDENTIAL INFORMATION
                       AND INVENTION ASSIGNMENT AGREEMENT


        As a condition of my employment with OraVax, Inc., its subsidiaries,
affiliates, successors or assigns (together, the "Company"), and in
consideration of my employment with the Company and my receipt of the
compensation now and hereafter paid to me by Company, I agree to the following:

         1.   CONFIDENTIAL INFORMATION.

              (a)   INFORMATION. I agree at all times during the term of my
employment and thereafter, to hold in strictest confidence, and not to use,
except for the benefit of the Company, or to disclose to any person, firm or
corporation without written authorization of the Board of Directors of the
Company, any Confidential Information of the Company. I understand that
"Confidential Information" means any Company proprietary information, technical
data, trade secrets or know-how, including, but not limited to, research,
product plans, products, projects, services, customer lists and customers
(including, but not limited to, customers of the Company on whom I called or
with whom I became acquainted during the term of my employment), markets,
software, developments, inventions, processes, methods, techniques, formulas,
compositions, compounds, technology, designs, drawings, research data, clinical
data, hardware configuration information, marketing, finances or other business
information disclosed to me by the Company either directly or indirectly in
writing, orally or by drawings or observation of parts or equipment. I further
understand that Confidential Information does not include any of the foregoing
items which has become publicly known and made generally available through no
wrongful act of mine or of others who were under confidentiality obligations as
to the item or items involved.

              (b)   FORMER EMPLOYER INFORMATION. I agree that I will not, during
my employment with the Company, improperly use or disclose any proprietary
information or trade secrets of any former or concurrent employer or other
person or entity and that I will not bring onto the premises of the Company any
unpublished documents or proprietary information belonging to any such employer,
person or entity unless consented to in writing by such employer, person or
entity.

              (c)   THIRD PARTY INFORMATION. I recognize 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 it only for certain limited
purposes. I agree to hold all such confidential or proprietary information in
the strictest confidence during the term of my employment and thereafter and not
to disclose it to any person, firm or corporation or to use it except as
necessary in carrying out my work for the Company consistent with the Company's
agreement with such third party.

        2.    INVENTIONS.

              (a)   INVENTIONS RETAINED AND LICENSED. I have attached hereto, as
Exhibit A, a list describing all inventions, original works of authorship,
developments, improvements, and trade secrets which were made by me prior to my
employment with the Company (collectively referred to as "Prior Inventions"),
which belong to me, which relate to the Company's proposed business, products or
research and development, and which are not assigned to the Company hereunder;
or, if no such list is attached, I represent that there are no such Prior
Inventions. If in the course of my employment with the Company, I incorporate
into a Company product, process or machine a Prior Invention owned by me or in
which I have an interest, the Company


<PAGE>   10

is hereby granted and shall have a nonexclusive, royalty-free, irrevocable,
perpetual, worldwide license to make, have made, modify, use and sell such Prior
Invention as part of or in connection with such product, process or machine.

              (b)   ASSIGNMENT OF INVENTIONS. I agree that I will promptly make
full written disclosure to the Company, will hold in trust for the sole right
and benefit of the Company, and hereby assign to the Company, or its designee,
all my right, title, and interest in and to any and all inventions, original
works of authorship, developments, discoveries, methods, concepts, improvements
or trade secrets, whether or not patentable or registrable under copyright or
similar laws, and all related patents, patent applications, copyrights and
copyright applications which I may solely or jointly conceive or develop or
reduce to practice, or cause to be conceived or developed or reduced to
practice, during the period of time I am in the employ of the Company, whether
or not during normal working hours or on the premises of the Company
(collectively referred to as ("Invention")). I further acknowledge that all
original works of authorship which are made by me (solely or jointly with
others) within the scope of and during the period of my employment with the
Company and which are protectible by copyright are "works made for hire," as
that term is defined in the United States Copyright Act. This Section 2(b) shall
not apply to Inventions which do not relate to the present or planned research
and development of the Company and which are made and concerned by the Employee
not during normal working hours, not on the Company's premises and not using the
Company's tools, devices, equipment or Proprietary Information.

              (c)   INVENTIONS ASSIGNED TO THE UNITED STATES. I agree to assign
to the United States government all my right, title, and interest in and to any
and all Inventions whenever such full title is required to be in the United
States by a contract between the Company and the United States or any of its
agencies.

              (d)   MAINTENANCE OF RECORDS. I agree to keep and maintain
adequate and current written records of all Inventions made by me (solely or
jointly with others) during the term of my employment with the Company. The
records will be in the form of notes, sketches, drawings, and any other format
that may be specified by the Company. The records will be available to and
remain the sole property of the Company at all times.

              (e)   PATENT AND COPYRIGHT REGISTRATIONS. I agree to assist the
Company, or its designee, at the Company's expense, in every proper way to
secure the Company's rights in the Inventions and any copyrights, patents, mask
work rights or other intellectual property rights relating thereto in any and
all countries, including the disclosure to the Company of all pertinent
information and data with respect thereto, the execution of all applications,
specifications, oaths, assignments and all other instruments which the Company
shall deem necessary in order to apply for and obtain such rights and in order
to assign and convey to the Company, its successors, assigns and nominees the
sole and exclusive rights, title and interest in and to such Inventions, and any
copyrights, patents, mask work rights or other intellectual property rights
relating thereto. I further agree that my obligation to execute or cause to be
executed, when it is in my power to do so, any such instrument or papers shall
continue after the termination of this Agreement. If the Company is unable
because of my mental or physical incapacity or for any other reason to secure my
signature to apply for or to pursue any application for any United States or
foreign patents or copyright registrations covering Inventions or original works
of authorship assigned to the Company as above, then I hereby irrevocably
designate and appoint the Company and its duly authorized officers and agents as
my agent and attorney-in-fact, to act for an in my 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 letters patent or copyright
registrations thereon with the same legal force and effect as if executed by me.


<PAGE>   11

         3.   RETURNING COMPANY DOCUMENTS. I agree that, at the time of leaving
the employ of the Company, I will deliver to the Company (and will not keep in
my possession, recreate or deliver to anyone else) any and all devices, records,
data, notes, reports, proposals, lists, correspondence, specifications,
drawings, blueprints, sketches, materials, equipment, other documents or
property, or reproductions of any aforementioned items developed by me pursuant
to my employment with the Company or otherwise belonging to the Company, its
successors or assigns. In the event of the termination of my employment, I agree
to sign and deliver the "Termination Certification" attached hereto as EXHIBIT
B.

         4.    NOTIFICATION TO NEW EMPLOYER.  In the event that I leave the
employ of the Company, I hereby grant consent to notification by the Company to
my new employer about my rights and obligations under this Agreement.

         5.   REPRESENTATIONS. I agree to execute any proper oath or verify any
proper document required to carry out the terms of this Agreement. I represent
that my performance of all the terms of this Agreement will not breach any
agreement to keep in confidence proprietary information acquired by me in
confidence or in trust prior to my employment by the Company. I have not entered
into, and I agree I will not enter into, any oral or written agreement in
conflict herewith.

         6.   GENERAL PROVISIONS

              (a)   GOVERNING LAW. This Agreement will be governed by the laws
of the Commonwealth of Massachusetts.

              (b)   ENTIRE AGREEMENT. This Agreement sets forth the entire
agreement and understanding between the Company and me relating to the subject
matter herein and merges all prior discussions between us. No modification of or
amendment to this Agreement, nor any waiver of any rights under this agreement,
will be effective unless in writing signed by the party to be charged. No delay
or omission by the Company in exercising any right under this Agreement will
operate as a waiver of this or any other right. A waiver or consent given by the
Company on any one occasion is effective only in that instance and will not be
construed as a bar or waiver of any right on any other occasion. Any subsequent
change or changes in my duties, salary or compensation will not affect the
validity or scope of this Agreement.

              (c)   SEVERABILITY. If one or more of the provisions in this
Agreement are deemed void by law, then the remaining provisions will continue in
full force and effect.

              (d)   SUCCESSORS AND ASSIGNS. This Agreement will be binding upon
my heirs, executors, administrators and other legal representatives and will be
for the benefit of the Company, its successors, and its assigns.

              (e)   The restrictions contained in this Agreement are necessary
for the protection of the business and goodwill of the Company and are
considered by you to be reasonable for such purpose. You agree that any breach
of this Agreement will cause the Company substantial and irrevocable damage and
therefore, in the event of any such breach, in addition to such other remedies
which may be available, the Company shall have the right to seek specific
performance and injunctive relief.




<PAGE>   12

         IN WITNESS WHEREOF, this Agreement has been executed by the parties
hereto as of the date set forth below.


Date: ____________________________           ___________________________________
                                             Signature



                                             ___________________________________
                                             Name of Employee (typed or printed)

Acknowledged and
Agreed to:


_______________________________
by: Lance K. Gordon, President
    OraVax, Inc.







<PAGE>   13

                                    EXHIBIT A

                            LIST OF PRIOR INVENTIONS
                        AND ORIGINAL WORKS OF AUTHORSHIP



                                                         Identifying Number
                 Title                        Date      Of Brief Description
                 -----                        ----      --------------------




_____ No inventions or improvements

_____ Additional Sheets Attached


Signature of Employee:___________________

Print Name of Employee:__________________


Date: _____________________




<PAGE>   14

                                    EXHIBIT B

                                  ORAVAX, INC.

                            TERMINATION CERTIFICATION


        This is to certify that I do not have in my possession, nor have I
failed to return, any devices, records, data, notes, reports, proposals, lists,
correspondence, specifications, drawings, blueprints, sketches, materials,
equipment, other documents or property, or reproductions of any aforementioned
items belonging to OraVax, Inc., its subsidiaries, affiliates, successors or
assigns (together, the "Company").

        I further certify that I have complied with all the terms of the
Company's Employee Confidential Information and Invention Assignment Agreement
signed by me, including the reporting of any inventions and original works of
authorship (as defined therein), conceived or made by me (solely or jointly with
others) covered by that agreement.

        I further agree that, in compliance with the Employee Confidential
Information and Invention Assignment Agreement, I will preserve as confidential
all trade secrets, confidential knowledge, data or other proprietary information
relating to products, processes, know-how, designs, formulas, developmental or
experimental work, computer programs, data bases, other original works of
authorship, customer lists, business plans, financial information or other
subject matter pertaining to any business of the Company or any of its
employees, clients, consultants or licensees.

        I further agree that for one year from this date, I will not hire any
employees of the Company and I will not solicit, induce, recruit or encourage
any of the Company's employees to leave their employment.

Date:


                                                    ____________________________
                                                    (Employee's Signature)


                                                    ____________________________
                                                    (Type/Print Employee's Name)





<PAGE>   15

                                   SCHEDULE C

                             CONSULTING ENGAGEMENTS


         Notwithstanding the provisions of Section 2 of this Agreement, the
Employee shall be permitted to act as an advisor to his former employer,
Immulogic Pharmaceutical Corporation, after the date of this Agreement and to
perform other consulting services, from time to time, with businesses not in
competition with the Company, subject to the approval of the Company's
President. In performing such permitted consulting services, the Employee (i)
must not engage in activities which would violate the terms of Section 6(a) of
this Agreement, and (ii) must schedule his consulting work so as not to
interfere with the performance of his duties for the Company.