<PAGE>
 
                                                                   Exhibit 10.13







                                        December 12, 1997





Anu D. Saad, Ph.D.
IMPATH Inc.
1010 Third Avenue
New York, New York  10021

Dear Anu:

     In consideration  of your service to IMPATH Inc. (the  "Company"),  you and
the Company agree as follows:

     1. In connection with any merger or  consolidation  in which the Company is
not  the  surviving  corporation  and  which  results  in  the  holders  of  the
outstanding  voting securities of the Company  (determined  immediately prior to
such merger or  consolidation)  owning  less than a majority of the  outstanding
voting securities of the surviving corporation (determined immediately following
such merger or consolidation),  or any sale or transfer by the Company of all or
substantially  all its assets or any tender  offer or exchange  offer for or the
acquisition, directly or indirectly, by any person or group of all or a majority
of the then outstanding voting securities of the Company,  all stock options for
the  purchase of common  stock of the Company  awarded to you prior to September
12, 1997  ("Options")  shall become  exercisable  in full,  notwithstanding  any
provision of the stock option plan of the Company  pursuant to which the Options
were granted or of the stock option  agreements or  certificates  evidencing the
Options,  on and after (i) the fifteenth day prior to the effective date of such
merger,  consolidation,  sale,  transfer  or  acquisition  or (ii)  the  date of
commencement  of such tender offer or exchange  offer,  as the case may be. With
respect to any Options which are incentive stock options,  the provisions of the
foregoing  sentence shall apply to the extent permitted by Section 422(d) of the
Code and such options in excess thereof shall,  immediately  upon the occurrence
of the event  described  in clause  (i) or (ii) of the  foregoing  sentence,  be
treated for all purposes as non-qualified stock options and shall be immediately
exercisable as such as provided in the foregoing sentence.
<PAGE>
 
                                                                               2



     2. In the event that your  employment  with the Company shall be terminated
by the Company  without Cause (as hereinafter  defined),  and not as a result of
your death or  Disability  (as  hereinafter  defined),  the Company shall make a
severance  payment  to  you  in an  amount  equal  to  one  year's  base  salary
(determined at your highest  annualized rate of base salary in effect during the
one-year  period  ending  on the  date of  termination),  payable  in  bi-weekly
installments during the one-year period commencing on the date of termination of
your  employment.  You shall be under no obligation to seek other  employment or
otherwise to mitigate the Company's obligation to make such severance payment to
you;  provided,  however,  that if you do obtain another position (whether as an
employee,  consultant,  partner or otherwise)  during such one-year period,  the
Company  shall  have the right to offset  against  such  severance  payment  any
salary,  fees,  bonus or other cash  compensation  actually earned by you during
such one-year  period from such other position.  The Company shall,  during such
one-year period,  continue to provide you with health insurance  benefits on the
same basis,  including  Company-paid  premiums, as such benefits are provided to
employees of the Company. Your rights under the other benefit plans and programs
of the Company shall be  determined  in accordance  with the terms of such plans
and programs as then in effect.

     For purposes of this  agreement,  a termination of your employment with the
Company by you for Good  Reason (as  hereinafter  defined)  shall  constitute  a
termination of your employment by the Company without Cause.

     For purposes of this  agreement:  "Cause" shall mean (a) your gross neglect
or willful  misconduct in the discharge of your duties and  responsibilities  to
the  Company,  (b)  your  material  and  repeated  failure  to obey  appropriate
directions  from the Board of  Directors  of the Company  which  failure has the
effect of  materially  injuring  the business or business  relationships  of the
Company,  (c) any act of willful  misappropriation by you against the Company or
(d) your  indictment,  conviction  or plea of  guilty  or nolo  contendere  with
respect to a felony crime; "Good Reason" shall mean (a) a reduction in your base
salary from the  annualized  rate in effect on the date  hereof or as  hereafter
increased or (b) a demotion in your  position with the Company or change in your
duties and  responsibilities  inconsistent with your position,  which reduction,
demotion or change shall not have been  corrected by the Company within ten (10)
days following notice thereof by you to the Company; and "Disability" shall mean
your failure by reason of sickness, accident or physical or mental disability to
substantially perform the duties and
<PAGE>
 
                                                                               3


responsibilities  of your  employment  with the  Company for a period of six (6)
months in any period of twelve (12) consecutive months.

     3. You agree that, in  consideration  of your  employment with the Company,
you will  not,  during  the  period  of your  employment  with the  Company  and
thereafter for a period of one (1) year commencing on the date of termination of
your employment with the Company, (a) engage, directly or indirectly, whether as
principal, agent, distributor,  representative,  consultant,  employee, partner,
stockholder,  limited partner or other investor (other than an investment of not
more than (i) five  percent (5%) of the stock or equity of any  corporation  the
capital  stock of which is  publicly  traded  or (ii) five  percent  (5%) of the
ownership interest of any limited partnership or other entity) or otherwise,  in
any business in  competition  with the business then conducted by the Company or
any of the Company's  subsidiaries (the Company and the Company's  subsidiaries,
being  hereinafter  collectively  referred to as the  "Company  Group"),  or (b)
solicit or entice or  endeavor  to solicit or entice away from any member of the
Company Group any person who was an employee of any member of the Company Group,
either  for your own  account or for any  individual,  firm or  corporation,  or
employ,  directly  or  indirectly,  any  person  who was during the one (1) year
period ending on the date of termination  of your  employment an employee of any
member of the Company Group.

     4. In the  event  of a breach  or  threatened  breach  by you of any of the
provisions of Section 3 of this agreement, you hereby consent and agree that the
Company shall (i) be entitled to cease  payment of the severance  referred to in
Section 2 of this  agreement  and (ii) be entitled to an  injunction  or similar
equitable relief from any court of competent  jurisdiction  restraining you from
committing  or  continuing  any such  breach or  threatened  breach or  granting
specific  performance  of any act  required to be  performed by you under any of
such  provisions,  without the  necessity  of showing any actual  damage or that
money damages  would not afford an adequate  remedy and without the necessity of
posting  any bond or other  security.  Nothing  herein  shall  be  construed  as
prohibiting  the Company from  pursuing  any other  remedies at law or in equity
which it may have with respect to any such breach or threatened breach.

     5.  This  agreement  shall be deemed a  contract  made  under,  and for all
purposes  shall be construed in  accordance  with,  the laws of the State of New
York applicable to contracts to be performed entirely within such State.
<PAGE>
 
                                                                               4


     6. This  agreement  contains  all the  understandings  and  representations
between the parties hereto pertaining to the subject matter hereof.

     7. No provision of this  agreement  may be amended or modified  unless such
amendment  or  modification  is agreed to in writing  and signed by you and by a
duly authorized representative of the Company.

     8. Should any provision of this agreement be held by a court or arbitration
panel of competent jurisdiction to be enforceable only if modified, such holding
shall not affect the validity of the remainder of this agreement, the balance of
which  shall  continue  to be  binding  upon the  parties  hereto  with any such
modification to become a part hereof and treated as though  originally set forth
in this agreement.  The parties further agree that any such court or arbitration
panel is expressly authorized to modify any such unenforceable provision of this
agreement in lieu of severing such  unenforceable  provision from this agreement
in its entirety,  whether by rewriting the offending provision,  deleting any or
all of the offending provision, adding additional language to this agreement, or
by making such other modifications as it deems warranted to carry out the intent
and agreement of the parties as embodied herein to the maximum extent  permitted
by law. The parties  expressly  agree that this  agreement as so modified by the
court or arbitration panel shall be binding upon and enforceable against each of
them. In any event,  should one or more of the  provisions of this  agreement be
held to be invalid,  illegal or unenforceable  in any respect,  such invalidity,
illegality or unenforceability shall not affect any other provisions hereof, and
if such  provision  or  provisions  are not  modified  as provided  above,  this
agreement  shall be  construed  as if such  invalid,  illegal  or  unenforceable
provisions had never been set forth herein.

     9. Anything to the contrary  notwithstanding,  all payments  required to be
made by the Company  hereunder  shall be subject to  withholding of such amounts
relating to taxes as the Company may  reasonably  determine  it should  withhold
pursuant to any applicable law or regulation.

                                    *  *  *
<PAGE>
 
                                                                               5


     Please  indicate your  acceptance  of and  agreement  with the foregoing by
signing and  returning  this  agreement  to the  Company,  whereupon  this shall
constitute a binding agreement between you and the Company.


                                            Very truly yours,

                                            IMPATH INC.



                                            By /s/ John P. Gandolfo
                                               --------------------



Accepted and Agreed:



     /s/ Anu D. Saad
     ---------------
         Anu D. Saad