<PAGE>
 
THIS NOTE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED
(THE "ACT"), OR THE SECURITIES LAWS OF ANY STATE, IN RELIANCE UPON EXEMPTIONS
FROM THE REGISTRATION REQUIREMENTS OF THE ACT AND SUCH LAWS.  THIS NOTE IS BEING
ACQUIRED BY THE PAYEE HEREOF FOR INVESTMENT ONLY AND FOR SAID PAYEE'S OWN
ACCOUNT, AND NEITHER THIS NOTE NOR ANY INTEREST HEREIN MAY BE OFFERED FOR SALE,
PLEDGED, HYPOTHECATED, SOLD, ASSIGNED OR TRANSFERRED AT ANY TIME EXCEPT IN
COMPLIANCE WITH THE ACT, ANY APPLICABLE STATE SECURITIES LAWS, AND ANY OTHER
LAWS WHICH ARE APPLICABLE TO SUCH TRANSACTION, AND IN COMPLIANCE WITH THE TERMS
AND CONDITIONS OF THIS NOTE.

THIS NOTE IS THE SENIOR SECURED NOTE REFERRED TO IN, AND IS ENTITLED TO THE
BENEFITS OF, THE SECURED LOAN AGREEMENT, THE SECURITY AGREEMENT, THE COLLATERAL
ASSIGNMENT OF LICENSE AGREEMENT AND THE PLEDGE AGREEMENT OF EVEN DATE HEREWITH
BETWEEN MAKER AND HOLDER (TOGETHER WITH THIS NOTE, THE "LOAN DOCUMENTS").


                              SENIOR SECURED NOTE

                             ---------------------

$2,000,000.00                                               December 6, 1996
                                                            Atlanta, Georgia

          FOR VALUE RECEIVED, the undersigned, ZYNAXIS, INC., a Pennsylvania
corporation ("Maker"), hereby promises to pay to the order of CYTRX CORPORATION,
a Delaware corporation (hereafter, together with any holder hereof, called
"Holder"), the principal sum of TWO MILLION DOLLARS ($2,000,000.00), or such
lesser amount as may be the aggregate principal amount of loans outstanding
hereunder, together with interest thereon as described below.

The Holder will provide to the undersigned monthly a statement of loans made,
charges incurred and payments made pursuant to this Note and the outstanding
amount of loans hereunder; provided, however, the failure of the Holder to
provide such statement shall in no way affect its rights or the undersigned's
obligations hereunder.
<PAGE>
 
     1.   Payment of Principal, Interest and Expenses.
          -------------------------------------------


          (a) Principal.  The aggregate principal balance of all loans
              ---------                                               
outstanding hereunder shall be due and payable at 5:00 p.m. on the first to
occur of:  (i) the day on which the closing of the transactions contemplated by
that certain Agreement and Plan of Merger and Contribution dated the date hereof
by and between Maker and Holder (the "Contribution Agreement") occurs, (ii) the
day on which the Contribution Agreement is terminated; or (iii) such other time
as may be agreed to in writing by Maker and Holder.

          (b)  Interest.
               -------- 

               (i)    Interest Rate.  Interest shall initially accrue on the
                      -------------                         
outstanding principal balance hereunder at a rate per annum equal to the rate
announced by NationsBank, N.A. (South) as its "prime rate" of interest (the
"Prime Rate") on the date hereof plus 200 basis points, and shall continue
accruing at such fixed rate until December 1, 1996, whereupon the interest rate
shall be adjusted to equal the Prime Rate in effect on such day plus 200 basis
points. Thereafter, the interest rate shall similarly be adjusted on the first
business day of every month to equal the Prime Rate in effect as of such day
plus 200 basis points. Interest shall be computed on the basis of a 360-day year
for the actual number of days elapsed. Holder shall notify Maker of the Prime
Rate within a reasonable time after the announcement thereof (A) on the date
hereof and (B) on any and all dates on which the interest rate shall be adjusted
in accordance with this paragraph 1(b)(i).

               (ii)   Calculation of Interest.  Interest shall be simple
                      -----------------------
interest.

               (iii)  Time of Payments.  Accrued interest shall be due and
                      ----------------                         
payable at such time as the principal amount hereof becomes due pursuant to
paragraph 1(a) above.

               (iv)   Interest on Past Due Amounts.  Interest shall accrue on
                      ----------------------------            
any amount past due hereunder at a rate equal to two percent (2.0%) per annum in
excess of the interest rate otherwise payable hereunder. All such interest shall
be due and payable on demand.

               (v)    Compliance with Usury Laws.  In no event shall the amount
                      --------------------------                
of interest due or payable under this Note exceed the maximum rate of interest
allowed by applicable law and, in the event any such payment is inadvertently
paid by the undersigned or inadvertently received by Holder, then such excess
sum shall be credited as a payment of principal, unless the undersigned shall
notify Holder in writing that the undersigned elects to have such excess sum
returned to it forthwith. It is the express intent of the parties hereto that
the undersigned not pay and Holder not receive, directly or indirectly, in any
manner whatsoever, interest in excess of that which may be lawfully paid by the
undersigned under applicable law.

                                      -2-
<PAGE>
 
THE UNDERSIGNED, AND HOLDER BY ACCEPTING THIS NOTE, EACH AGREE AND STIPULATE
THAT THE ONLY CHARGE IMPOSED UPON THE UNDERSIGNED FOR THE USE OF MONEY IN
CONNECTION WITH THIS NOTE IS AND SHALL BE THE INTEREST DESCRIBED ABOVE, AND
FURTHER AGREE AND STIPULATE THAT ALL OTHER CHARGES IMPOSED BY HOLDER ON THE
UNDERSIGNED IN CONNECTION WITH THIS NOTE, INCLUDING WITHOUT LIMITATION, ALL
DEFAULT CHARGES, LATE CHARGES, PREPAYMENT FEES AND ATTORNEYS' FEES, ARE CHARGES
MADE TO COMPENSATE HOLDER FOR UNDERWRITING OR ADMINISTRATIVE SERVICES AND COSTS
OR LOSSES PERFORMED OR INCURRED, AND TO BE PERFORMED OR INCURRED, BY HOLDER IN
CONNECTION WITH THIS NOTE AND SHALL UNDER NO CIRCUMSTANCES BE DEEMED TO BE
CHARGES FOR THE USE OF MONEY PURSUANT TO OFFICIAL CODE OF GEORGIA ANNOTATED
SECTION 7-4-2 OR SECTION 7-4-18.  ALL CHARGES OTHER THAN CHARGES FOR THE USE OF
MONEY SHALL BE FULLY EARNED AND NONREFUNDABLE WHEN DUE.

          (c) Expenses.  The undersigned shall pay all expenses incurred by
              --------                                                     
Holder in the collection of this Note, including, without limitation, the
reasonable fees and disbursements of counsel to Holder, if this Note is
collected by or through an attorney-at-law.

          (d) General.  All payments of principal, interest and expenses shall
              -------                                                         
be in lawful money of the United States of America, and made according to such
wire transfer instructions or other delivery method as Holder may designate to
Maker in writing from time to time.

    2.   Right of Set-Off.  In addition to any rights now or hereafter granted
         ----------------                                                     
under applicable law and not by way of limitation of any such rights, the
undersigned hereby authorizes Holder, at any time or from time to time, without
notice to the undersigned or to any other person or entity, any such notice
being hereby expressly waived, to set-off and to appropriate and to apply any
and all indebtedness at any time held or owing by Holder or any affiliate of
Holder, to or for the credit or the account of the undersigned, against and on
account of all obligations of the undersigned owing hereunder or otherwise to
Holder, irrespective of whether or not Holder shall have declared any or all of
such obligations of the undersigned to be due and payable, and although such
obligations shall be contingent or unmatured.

    3.   Waiver.  Maker, and its successors and assigns, waive presentment for
         ------                                                               
payment, demand, protest and notice of demand, dishonor, notice of dishonor,
protest and nonpayment.  In any action on this Note, Holder or its assignee need
not produce or file the original of this Note, but need only file a photocopy of
this Note certified by Holder or such assignee to be a true and correct copy of
this Note in all material respects.

                                      -3-
<PAGE>
 
    THE UNDERSIGNED, AND HOLDER BY ACCEPTING THIS NOTE, EACH ACKNOWLEDGES THAT
ANY DISPUTE OR CONTROVERSY BETWEEN THE UNDERSIGNED AND HOLDER WOULD BE BASED ON
DIFFICULT AND COMPLEX ISSUES OF LAW AND FACT.  ACCORDINGLY, HOLDER AND THE
UNDERSIGNED HEREBY WAIVE TRIAL BY JURY IN ANY ACTION OR PROCEEDING OF ANY KIND
OR NATURE IN ANY COURT OR TRIBUNAL IN WHICH AN ACTION MAY BE COMMENCED BY OR
AGAINST THE UNDERSIGNED ARISING OUT OF THIS NOTE OR BY REASON OF ANY OTHER CAUSE
OR DISPUTE WHATSOEVER BETWEEN THE UNDERSIGNED AND HOLDER OF ANY KIND OR NATURE.

    THE UNDERSIGNED, AND HOLDER BY ACCEPTING THIS NOTE, HEREBY AGREE THAT THE
FEDERAL COURT OF THE NORTHERN DISTRICT OF GEORGIA OR, AT THE OPTION OF HOLDER,
ANY STATE COURT LOCATED IN FULTON COUNTY, GEORGIA SHALL HAVE JURISDICTION TO
HEAR AND DETERMINE ANY CLAIMS OR DISPUTES BETWEEN THE UNDERSIGNED AND HOLDER
PERTAINING DIRECTLY OR INDIRECTLY TO THIS NOTE OR ANY OTHER CAUSE OR DISPUTE
WHATSOEVER BETWEEN THE UNDERSIGNED AND HOLDER OF ANY KIND OR NATURE.  THE
UNDERSIGNED EXPRESSLY SUBMITS AND CONSENTS IN ADVANCE TO SUCH JURISDICTION IN
ANY ACTION OR PROCEEDING COMMENCED IN SUCH COURTS, HEREBY WAIVING PERSONAL
SERVICE OF THE SUMMONS AND COMPLAINT, OR OTHER PROCESS OR PAPERS ISSUED THEREIN,
AND AGREEING THAT SERVICE OF SUCH SUMMONS AND COMPLAINT, OR OTHER PROCESS OR
PAPERS MAY BE MADE BY REGISTERED OR CERTIFIED MAIL, RETURN RECEIPT REQUESTED
ADDRESSED TO THE UNDERSIGNED AT THE ADDRESS OF THE UNDERSIGNED SET FORTH BELOW
ITS SIGNATURE HERETO.  SHOULD THE UNDERSIGNED FAIL TO APPEAR OR ANSWER ANY
SUMMONS, COMPLAINT, PROCESS OR PAPERS SO SERVED WITHIN THIRTY DAYS AFTER THE
MAILING THEREOF, IT SHALL BE DEEMED IN DEFAULT AND AN ORDER AND/OR JUDGMENT MAY
BE ENTERED AGAINST IT AS PRAYED FOR IN SUCH SUMMONS, COMPLAINT, PROCESS OR
PAPERS.  THE CHOICE OF FORUM SET FORTH IN THIS SECTION SHALL NOT BE DEEMED TO
PRECLUDE THE BRINGING OF ANY ACTION BY HOLDER OR THE ENFORCEMENT BY HOLDER OF
ANY JUDGMENT OBTAINED IN SUCH FORUM IN ANY OTHER APPROPRIATE JURISDICTION.
FURTHER, THE UNDERSIGNED HEREBY WAIVES THE RIGHT TO ASSERT THE DEFENSE OF FORUM
NON CONVENIENS AND THE RIGHT TO CHALLENGE THE VENUE OF ANY COURT PROCEEDING.

    THE UNDERSIGNED AGREES THAT ALL OF ITS PAYMENT OBLIGATIONS HEREUNDER SHALL
BE ABSOLUTE, UNCONDITIONAL AND, FOR THE PURPOSES OF MAKING PAYMENTS HEREUNDER,
THE UNDERSIGNED HEREBY WAIVES ANY RIGHT TO ASSERT ANY SETOFF, COUNTERCLAIM OR
CROSS-CLAIM.

                                      -4-
<PAGE>
 
    FURTHER, THE UNDERSIGNED WAIVES (I) ANY NOTICE OR HEARING PRIOR TO THE
TAKING POSSESSION OR CONTROL BY HOLDER OF ANY COLLATERAL GIVEN BY THE
UNDERSIGNED, (II) THE POSTING OF ANY BOND OR SECURITY WHICH MIGHT BE REQUIRED BY
ANY COURT PRIOR TO ALLOWING HOLDER TO EXERCISE ANY OF ITS RIGHTS OR REMEDIES,
INCLUDING THE ISSUANCE OF AN IMMEDIATE WRIT OF POSSESSION AND (III) THE BENEFIT
OF ALL VALUATION, APPRAISEMENT AND EXEMPTION OF LAWS.

    THE FOREGOING WAIVERS HAVE BEEN MADE WITH THE ADVICE OF COUNSEL AND WITH A
FULL UNDERSTANDING OF THE LEGAL CONSEQUENCES THEREOF.

    4.   Governing Law.  This Note shall be construed, interpreted and enforced
         -------------                                                         
in accordance with the laws of the State of Georgia, without regard to the law
of the conflicts of laws of such State.

    5.   Return of Note.  This Note shall be returned to Maker upon the payment
         --------------                                                        
in full of all amounts owed under this Note.

    6.   Default.  Each of the following events shall constitute an "Event of
         -------                                                             
Default" under this Note:  (i) Maker shall fail to pay any principal, interest
or other amount due hereunder when due, or Maker shall in any way fail to comply
with the other terms, covenants or conditions contained in this Note or the
other Loan Documents; (ii) any written representation or warranty made at any
time by Maker to the Holder shall prove to have been incorrect or misleading in
any material respect when made; (iii) a default, event of default, or event
which with the giving of notice or the passage of time or both would constitute
a default or event of default, shall have occurred under any other document,
instrument, contract or agreement now or hereafter entered into by Maker and
Holder or executed by Maker in favor of the Holder, or Maker shall in any
material way fail to comply with the terms, covenants or conditions contained in
any such document, instrument, contract or agreement; (iv) a default, event of
default, or event which with the giving of notice or the passage of time or both
would constitute a default or event of default, shall have occurred under any
document, instrument, contract or agreement (a) evidencing or securing
indebtedness of Maker for borrowed money or (b) material to the financial
condition of Maker; (v) a final judgment or order for the payment of money, or
any final order granting equitable relief, shall be entered against Maker and
such judgment or order has or will have a materially adverse effect on the
financial condition of Maker; (vi) a warrant, writ of attachment, levy or other
similar process shall be issued against any property of Maker; (vii) Maker shall
(a) commence a voluntary case under the Bankruptcy Code of 1978, as amended or
other federal bankruptcy law (as now or hereafter in effect); (b) file a
petition seeking to take advantage of any other laws, domestic or foreign,
relating to bankruptcy, insolvency, reorganization, winding up or composition
for adjustment of debts; (c) consent to or fail to contest in a timely and

                                      -5-
<PAGE>
 
appropriate manner any petition filed against it in an involuntary case under
such bankruptcy laws or other laws; (d) apply for or consent to, or fail to
contest in a timely and appropriate manner, the appointment of, or the taking of
possession by, a receiver, custodian, trustee, or liquidator of itself or of a
substantial part of its property, domestic or foreign; (e) be unable to, or
admit in writing its inability to, pay its debts as they become due; (f) make a
general assignment for the benefit of creditors; or (g) make a conveyance
fraudulent as to creditors under any state or federal law; or (viii) a case or
other proceeding shall be commenced against Maker in any court of competent
jurisdiction seeking (a) relief under the Bankruptcy Code of 1978, as amended or
other federal bankruptcy law (as now or hereafter in effect) or under any other
laws, domestic or foreign, relating to bankruptcy, insolvency, reorganization,
winding up or adjustment of debts or (b) the appointment of a trustee, receiver,
custodian, liquidator or the like for Maker or all or any substantial part of
the assets, domestic or foreign, of Maker.

    Upon the occurrence of an Event of Default (other than an Event of Default
described in clause (vii) or (viii) of the definition thereof), any and all of
the loans and the undersigned's other obligations hereunder, at the option of
the Holder, and without demand or notice of any kind, may be immediately
declared, and thereupon shall immediately become in default and due and payable
and the Holder may exercise any and all rights and remedies available to it
under the Loan Documents, at law, in equity or otherwise.  Upon the occurrence
of an Event of Default described in clause (vii) or (viii) of the definition
thereof, any and all of the loans and the undersigned's other obligations
hereunder, without demand or notice of any kind, shall immediately become in
default and due and payable and the Holder may exercise any and all rights and
remedies available to it under the Loan Documents, at law, in equity or
otherwise.

    7.   Representations and Warranties; Covenants and Agreements.  The
         --------------------------------------------------------      
representations and warranties made in Article 5 of the Contribution Agreement
by Maker and the agreements of Maker made in Articles 7 and 8 of the
Contribution Agreement are incorporated herein by this reference; provided,
however, that the affirmative and negative covenants shall survive termination
of the Contribution Agreement until such time as the principal of this note and
all interest accrued thereon have been paid in full by Maker.

    8.   Miscellaneous.
         ------------- 

         (a) To the extent that it becomes necessary for Holder to engage legal
counsel to enforce the obligations of Maker to Holder hereunder, Maker shall pay
all reasonable, necessary, and actually incurred expenses of such legal counsel.

         (b) Time is of the essence of this Note.

         (c) All amendments to this Note, and any waiver or consent of Holder,
must be in writing and signed by Holder and Maker.

                                      -6-
<PAGE>
 
         (d) This Note shall be binding upon the successors and assigns of
Maker.

    IN WITNESS WHEREOF, the undersigned has caused this Note to be duly executed
and delivered under seal as of the day and year first above written.

                             ZYNAXIS, INC.



                             By:    /s/ Martyn Greenacre
                                --------------------------------------
                                Name:      M.D. Greenacre
                                     ---------------------------------
                                Title:     Chairman, President & CEO
                                      --------------------------------


                             ATTEST:

                             By:    /s/ Michael A. Christie
                                --------------------------------------
                                Name:      Michael A. Christie
                                     ---------------------------------
                                Title:     Secretary
                                      --------------------------------


                             Address: 371 Phoenixville Pike
                                      Malvern, Pennsylvania  19355

                                      -7-