<PAGE>
 
                   COLLATERAL ASSIGNMENT OF LICENSE AGREEMENT


     THIS COLLATERAL ASSIGNMENT OF LICENSE AGREEMENT dated as of December 6,
1996, executed and delivered by ZYNAXIS VACCINE TECHNOLOGIES, INC., a
Pennsylvania corporation (the "Debtor"), in favor of CYTRX CORPORATION, a
Delaware corporation (the "Secured Party").

     WHEREAS, the Secured Party is extending a secured loan (the "Secured Loan")
to Zynaxis, Inc., a Pennsylvania corporation and the Debtor's parent
("Zynaxis"), which Secured Loan will be guaranteed by the Debtor pursuant to
that certain Guaranty dated as of the date hereof (the "Guaranty");

     WHEREAS, the Debtor is party to a certain License Agreement dated July 1,
1987 (as amended, supplemented, restated or otherwise modified from time to
time, the "Assigned Agreement") with Southern Research Institute ("Southern"),
pursuant to which the Debtor licenses certain technology from Southern; and

     WHEREAS, as a condition to the Secured Party extending the Secured Loan to
Zynaxis, the Secured Party has required, among other things, that the Debtor
execute and deliver the Guaranty and, in connection therewith, assign to the
Secured Party, and grant to the Secured Party a security interest in, the
Debtor's right, title and interest in, to and under the Assigned Agreement, as
additional security for the Guaranteed Obligations.

     NOW, THEREFORE, in consideration of the premises set forth herein and for
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged by the Debtor, the Debtor agrees as follows:

     Section 1.  Assignment and Security Interest.  As security for the payment
                 --------------------------------                              
and performance of the Guaranteed Obligations, the Debtor hereby assigns to the
Secured Party, and grants to the Secured Party a security interest in (a) all of
the Debtor's right, title and interest in, to and under the Assigned Agreement,
together with all other documents, instruments, agreements, certificates and
opinions delivered in connection with the Assigned Agreement, all as the same
may be amended, supplemented, restated or otherwise modified from time to time,
(the "Assigned Agreement Documents"), including without limitation, (i) all
rights of the Debtor to receive moneys due and to become due to it thereunder or
in connection therewith; (ii) all rights of the Debtor to damages arising out
of, or for, breach or default in respect thereof and (iii) all rights of the
Debtor to perform and exercise all rights and remedies thereunder; (b) all of
the Debtor's books and records in any way relating to the Assigned Agreement
Documents and (c) all products and proceeds of any of the foregoing (all of the
foregoing, collectively the "Collateral").
<PAGE>
 
     Section 2.  Covenants.  The Debtor covenants and agrees as to each Assigned
                 ---------                                                      
Agreement Document as follows:

     (a) The Debtor shall at all times duly and punctually perform all of the
terms, conditions and covenants of the Assigned Agreement Documents on the
Debtor's part to be kept, observed and performed.

     (b) The Debtor will not sell, assign, grant a Lien in, or otherwise
transfer to any Person any of the Debtor's right, title or interest in, to or
under any of the Collateral.

     (c) The Debtor will not amend or otherwise modify any of the terms of any
Assigned Agreement Document, or terminate any Assigned Agreement Document,
without the Secured Party's prior written consent.

     (d) The Debtor shall promptly deliver to the Secured Party a copy of any
notice or other written communication given by Southern to the Debtor or by the
Debtor to Southern under or in connection with any Assigned Agreement Document.

     Section 3.  Representations and Warranties.  The Debtor represents and
                 ------------------------------                            
warrants to the Secured Party as follows:

     (a) True, correct and complete copies of the Assigned Agreement Documents
(including all amendments, supplements and modifications thereto) have been
delivered to the Secured Party.

     (b) Each Assigned Agreement Document is in full force and effect and is the
legal, valid and binding obligation of the Debtor and Southern, enforceable
against the Debtor and Southern in accordance with its terms.

     (c) With the exception of the Debtor's failure to pay royalties as
described in letters from Southern to Debtor dated August 21, 1996 and August
27, 1996 (the "Royalty Default"), neither the Debtor nor Southern is in default
under any Assigned Agreement Document.

     (d) Except for that certain Development and License Agreement by and
between Zynaxis and ALK A/S dated September 21, 1995, the Debtor has not
assigned any of its right, title or interest in, to or under any of the
Collateral to any Person.

     (e) The Debtor has full authority and the legal right to assign to the
Secured Party, and to grant to the Secured Party a security interest in, the
Assigned Agreement Documents and the other Collateral and the Debtor has
obtained all consents necessary for the valid and binding assignment of and the
effectiveness and enforceability of such security interest under this Agreement.

     (f) All of the Collateral is free and clear of all Liens.

                                      -2-
<PAGE>
 
     Section 4.  Debtor's Right to Enforce.  So long as no Event of Default
                 -------------------------                                 
shall have occurred, the Secured Party will not exercise or enforce, or seek to
exercise or enforce, or avail itself of, any of the rights, powers, privileges,
authorizations and benefits assigned and transferred to the Secured Party
pursuant to this Agreement, and the Debtor may exercise or enforce, or seek to
exercise or enforce, such rights, powers, privileges, authorizations and
benefits in conformity with the provisions of this Agreement.  If the Debtor
shall fail to perform or comply with any term or condition imposed upon the
Debtor under any of the Assigned Agreement Documents, or the Debtor shall fail
to exercise any right or remedy of the Debtor thereunder, then, without waiving
or releasing the Debtor from any of its obligations hereunder or under such
Assigned Agreement Document, the Secured Party may (but shall not obligated to)
take any action the Secured Party deems necessary or desirable to preserve and
protect any of the Debtor's rights thereunder or the Secured Party's rights
thereunder; provided, however, that the Secured Party may not take any such
action if the Debtor complies with the covenant in Section 8.10 of the Agreement
and Plan of Merger and Contribution dated the date hereof to which the Debtor
and the Secured Party are parties.

     Section 5.  Remedies.  Upon the occurrence of an Event of Default, the
                 --------                                                  
Secured Party shall, at its election, without notice of election and without
demand, have the right to do any one or more of the following acts, all of which
are hereby authorized by the Debtor: (a) exercise any or all of the rights
available to a secured party under the Uniform Commercial Code or any other
Applicable Law; (b) exercise any or all of its rights and remedies under the
Loan Documents; and (c) through Vaxcel, Inc., a Delaware corporation and a
Subsidiary of the Secured Party, exercise any or all of the Debtor's rights
under any or all of the Assigned Agreement Documents to the exclusion of the
Debtor.

     Section 6.  Secured Party Not Obligated.  Notwithstanding any other
                 ---------------------------                            
provision of this Agreement to the contrary, the Debtor expressly acknowledges
and agrees that it shall continue to observe and perform all of the conditions
and obligations contained in the Assigned Agreement Documents to be observed and
performed by it, and that neither this Agreement, nor any action taken pursuant
hereto, shall cause the Secured Party to be under any obligation or liability in
any respect whatsoever to any party to any Assigned Agreement Document or to any
other Person for the observance or performance of any of the representations,
warranties, conditions, covenants, agreements or terms therein contained.

     Section 7.  Further Assurances.  At any time and from time to time, upon
                 ------------------                                          
the written request of the Secured Party, and at the sole expense of the Debtor,
the Debtor will promptly execute and deliver such further instruments and
documents and take such further action as the Secured Party may reasonably
request for the purpose of obtaining or preserving the full benefits of this
Agreement and of the rights and powers herein granted.

                                      -3-
<PAGE>
 
     Section 8.  Rights Cumulative.  The rights and remedies of the Secured
                 -----------------                                         
Party under this Agreement are cumulative and not exclusive of any rights or
remedies which it would otherwise have.  In exercising its rights and remedies
the Secured Party may be selective and no failure or delay by the Secured Party
in exercising any right shall operate as a waiver of it, nor shall any single or
partial exercise of any power or right preclude its other or further exercise or
the exercise of any other power or right.

     Section 9.  Secured Party Appointed Attorney-in-Fact.  The Debtor hereby
                 ----------------------------------------                    
irrevocably appoints the Secured Party as the Debtor's attorney-in-fact, with
full power of substitution, effective after the occurrence of an Event of
Default and for so long thereafter as there are unsatisfied Guaranteed
Obligations, with full authority in the place and stead of the Debtor and in the
name of the Debtor or otherwise, from time to time in the Secured Party's
discretion, to take any action and to execute any instrument or document which
the Secured Party may deem necessary or advisable to accomplish the purposes of
this Agreement and to exercise any rights and remedies the Secured Party may
have under this Agreement or Applicable Law.  The power-of-attorney granted
hereby is irrevocable and coupled with an interest.

     Section 10.  Expenses.  The Debtor will pay, on demand, all reasonable out-
                  --------                                                     
of-pocket expenses incurred by the Secured Party or its assignee in connection
with: (a) the collection or enforcement of the Guaranteed Obligations including
the reasonable fees and disbursements of counsel to the Secured Party, if such
collection or enforcement is done through or by an attorney; and (b) the
exercise by the Secured Party or its assignee of any right or remedy granted to
it under this Agreement, if a Default or Event of Default has occurred.

     Section 11.  Amendments, Etc.  No amendment or waiver of any provision of
                  ---------------                                             
this Agreement, nor consent to any departure by the Debtor herefrom, shall in
any event be effective unless the same shall be in writing and signed by the
parties hereto, and then such waiver or consent shall be effective only in the
specific instance and for the specific purpose for which given.

     Section 12.  Notices.  Notices, requests and other communications required
                  -------                                                      
or permitted hereunder shall be given in accordance with the applicable terms of
the Security Agreement of even date herewith between the Secured Party and the
Debtor.

     Section 13. Continuing Security Interest.  This Agreement shall create a
                 ----------------------------                                
continuing security interest in the Collateral and shall (i) remain in full
force and effect until indefeasible payment in full of the Guaranteed
Obligations, (ii) be binding upon the Debtor, its successors and assigns and
(iii) inure to the benefit of the Secured Party, and its successors and assigns.
The Debtor's successors and assigns shall include, without limitation, a
receiver, trustee or debtor-in-possession thereof or therefore.

     Section 14.  Applicable Law; Severability.  THIS AGREEMENT SHALL BE
                  ----------------------------                          
GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF 

                                      -4-
<PAGE>
 
THE STATE OF GEORGIA. Whenever possible, each provision of this Agreement shall
be interpreted in such a manner as to be effective and valid under Applicable
Law, but if any provision of this Agreement shall be prohibited by or invalid
under Applicable Law, such provisions shall be ineffective only to the extent of
such prohibition or invalidity, without invalidating the remainder of such
provisions or the remaining provisions of this Agreement.

     Section 15.  Indemnification.  The Debtor agrees to indemnify and hold the
                  ---------------                                              
Secured Party and its officers, directors, employees, agents and Affiliates
harmless from and against any claim, loss, damage, action, cause of action,
liability, cost and expense or suit of any kind or nature whatsoever, including
reasonable attorneys' fees and expenses, brought against or incurred by the
Secured Party or its officers, directors, employees, agents and Affiliates in
any manner arising out of or, directly or indirectly, related to or connected
with this Agreement, including without limitation, the exercise by the Secured
Party or the Secured Party's assignee of any of its rights and remedies under
this Agreement or under any of the Assigned Agreement Documents or any other
action taken by the Secured Party or its assignee pursuant to the terms of this
Agreement; provided, however, that the indemnity set forth in this Section 15
           --------  -------                                                 
shall not apply to the extent that the claim, loss, damage, action, cause of
action, liability, cost, expense or suit arises primarily and directly from the
gross negligence or willful misconduct of the Secured Party or its assignee.

     Section 16.  Counterparts.  This Agreement may be executed in several
                  ------------                                            
counterparts, each of which shall be an original and all of which, taken
together, shall constitute but one and the same instrument.

     Section 17.  Benefits.  The terms, covenants and conditions contained
                  --------                                                
herein shall inure to the benefit of the Secured Party, and its successors and
assigns, and shall be binding on the Debtor and its successors and assigns.

     Section 18.  Definitions.  Capitalized terms not otherwise defined herein
                  -----------                                                 
are used herein as with the respective meanings given them in (i) the Security
Agreement of even date herewith between the Debtor and the Secured Party and
(ii) the Senior Secured Note of even date herewith executed by Zynaxis in favor
of the Secured Party, as the case may be.

                                      -5-
<PAGE>
 
     IN WITNESS WHEREOF, the Debtor has executed and delivered this Collateral
Assignment of License Agreement under seal as of the date first written above.

                                        ZYNAXIS VACCINE TECHNOLOGIES, INC.


                                        By:  /s/ Martyn Greenacre
                                           ---------------------------
                                           Title:  President
                                                 ---------------------

                                        ATTEST:


                                        By:
                                           ---------------------------------
                                           Title:
                                                 ---------------------------

                                                (CORPORATE SEAL)

                                      -6-
<PAGE>
 
                         ACKNOWLEDGMENT AND AGREEMENT

     The undersigned, Southern Research Institute ("Southern"), acknowledges
receipt of a copy of the within and foregoing Collateral Assignment of License
Agreement (the "Assignment").  Capitalized terms not otherwise defined herein
are used herein with the respective meanings given them in the Assignment.

     NOW, THEREFORE, for good and valuable consideration, including the economic
benefit expected to accrue to Southern Research Institute as a result of the
proposed transactions between the Debtor and the Secured Party, the receipt and
sufficiency of which are hereby acknowledged by Southern, Southern agrees as
follows:

     Section 1.  Consent to Assignment.  Southern consents to the execution and
                 ---------------------                                         
delivery of the Assignment by the Debtor and the assignment by the Debtor to the
Secured Party, and the grant by the Debtor to the Secured Party of a security
interest in, all of the Debtor's right, title and interest in, to and under the
Assigned Agreement Documents and the other Collateral upon satisfaction of the
following conditions:

     a.   the Collateral is subsequently assigned to Vaxcel, Inc., a wholly
          owned subsidiary of the Secured Party; and

     b.   either Debtor or the Secured Party satisfies the monetary obligations
          of the Debtor to Southern under the Assigned Agreement, including but
          not limited to: (A) Debtor's obligation to cure the Royalty Default
          and (B)  Debtor's obligations to pay expenses of filing, prosecuting,
          defending and maintaining patent applications and patents issued
          thereon.

     Upon receipt by the undersigned of: (i) written notice from the Secured
Party that an Event of Default has occurred under the Secured Loan, and (ii)
proof of satisfaction of the above conditions that is reasonably satisfactory to
the undersigned, the undersigned shall immediately thereupon, and at all times
thereafter, permit the Secured Party, at the Secured Party's option, to exercise
all of the rights and benefits of the Debtor under or in respect of such
Assigned Agreement Documents and the other Collateral which the Secured Party
desires to exercise, all to the complete and absolute exclusion of the Debtor.
The undersigned acknowledges and agrees that any exercise by the Secured Party
of any such rights and benefits shall not discharge or otherwise release the
Debtor from any of its obligations under or in respect of any of the Assigned
Agreement Documents and the other Collateral.

     Section 2.  Benefits.  This Acknowledgment shall be binding upon the
                 --------                                                
undersigned and its successors and assigns, and shall inure to the benefit of
the Secured Party and its successors and assigns.
<PAGE>
 
     IN WITNESS WHEREOF, the undersigned has executed and delivered this
Acknowledgment and Agreement under seal as of the date first written above.

                                        SOUTHERN RESEARCH INSTITUTE


                                        By: /s/   John Rouse
                                           -----------------
                                           Title:  President
                                                   ---------