<PAGE>
 
                               PLEDGE AGREEMENT


     THIS PLEDGE AGREEMENT dated as of December 6, 1996 by and between ZYNAXIS,
INC., a Pennsylvania corporation (the "Pledgor") and CYTRX CORPORATION, a
Delaware corporation (the "Pledgee").

     WHEREAS, Pledgee has agreed to lend funds to the Pledgor, with the
Pledgor's obligation to repay the loan being evidenced by that certain Senior
Secured Note made by Pledgor in favor of Pledgee that is being delivered
simultaneously with the execution of this Agreement (the "Note"); and

     WHEREAS, in connection with such loan the Pledgee requires a first priority
security interest in all of the assets of the Pledgor.

     NOW, THEREFORE, in consideration of the mutual agreements herein contained
and for other good and valuable consideration, the receipt and sufficiency of
which are hereby acknowledged, the parties hereto hereby agree as follows:

     Section 1.  Pledge.  The Pledgor hereby pledges, hypothecates, assigns,
                 ------                                                     
transfers, sets over and delivers unto the Pledgee, and grants to the Pledgee a
security interest in, all of the Pledgor's right, title and interest in, to and
under the following (collectively, the "Pledged Collateral"): (a) all of the
common stock, shares, equity interest and other securities (collectively,
"Securities") of each Person (each an "Issuer") described in Schedule 1 attached
                                                             ----------         
hereto; (b) any additional Securities of any of such Issuers as may from time to
time be issued to the Pledgor or otherwise acquired by the Pledgor; (c) any
additional Securities of any Issuer as may hereafter at any time be delivered to
the Pledgee by or on behalf of the Pledgor; (d) any cash or additional
Securities or other property at any time and from time to time receivable or
otherwise distributable in respect of, in exchange for, or in substitution of,
any of the property referred to in any of the immediately preceding clauses (a)
through (c); and (e) any and all products and proceeds of any of the foregoing,
together with any and all other rights, titles, interests, powers, privileges
and preferences pertaining to said property.

     Section 2.  Obligations Secured.  This Agreement is made, and the security
                 -------------------                                           
interest created hereby is granted to the Pledgee, to secure the prompt
performance and payment in full of all indebtedness, liabilities, obligations,
covenants and duties of the Pledgor owing to the Pledgee of every kind, nature
and description, whether direct or indirect, absolute or contingent, due or not
due, contractual or tortious, liquidated or unliquidated, and whether or not
evidenced by any note, including any reasonable fees of or costs or expenses
incurred by the Pledgee or Pledgee's counsel in connection with the realization
of the security for which this Agreement provides, including, without
limitation, any reasonable costs or expenses of any proceedings to which this
Agreement may give rise.
<PAGE>
 
     Section 3.  Representations and Warranties.  The Pledgor hereby represents
                 ------------------------------                                
and warrants to the Pledgee as follows:

     (a) Validly Issued, etc.  All of the Securities of each Issuer have been
         --------------------                                                
validly issued and are fully paid and nonassessable.

     (b) Title and Liens.  The Pledgor is, and will at all times continue to be,
         ---------------                                                        
the legal and beneficial owner of the Pledged Collateral and none of the Pledged
Collateral is subject to any Lien.  No financing statement under the Uniform
Commercial Code of any jurisdiction which names the Pledgor as debtor or covers
any of the Pledged Collateral, or any other notice filed in the public records
indicating the existence of a Lien thereon, has been filed and is still
effective in any state or other jurisdiction, other than Uniform Commercial Code
financing statements filed in favor of the Pledgee, and the Pledgor has not
signed any such financing statement or notice or any security agreement
authorizing the filing of any such financing statement or notice, other than
Uniform Commercial Code financing statements filed in favor of the Pledgee.

     (c) Name; Chief Executive Office; Taxpayer ID Number.  The correct
         ------------------------------------------------              
corporate name of the Pledgor is set forth in the first paragraph of this
Agreement.  The chief executive office and principal place of business of the
Pledgor and the location of the Pledgor's books and records relating to the
Pledged Collateral are located at 371 Phoenixville Pike, Malvern, Chester
County, Pennsylvania.  The Internal Revenue Service taxpayer identification
number of the Pledgor is 23-2562913.

     (d) Authority, etc.  The Pledgor (i) has the power and authority to pledge
         --------------                                                        
the Pledged Collateral in the manner hereby done or contemplated and (ii) will
defend its title or interest thereto or therein against any and all Liens (other
than the Lien created by this Agreement and Permitted Liens), however arising,
of all Persons.

     (e) No Approval.  No consent or approval of any Governmental Authority or
         -----------                                                          
any securities exchange was or is necessary to the validity of the pledge
effected hereby.

     (f) Outstanding Shares.  Schedule 2 attached hereto sets forth the
         ------------------                                            
authorized capital structure of each Issuer and the issued and outstanding
shares of stock of each class of capital stock.

     Section 4.  Covenants.  The Pledgor hereby unconditionally covenants and
                 ---------                                                   
agrees as follows:

     (a) No Liens; No Sale of Pledged Collateral.  The Pledgor will not create,
         ---------------------------------------                               
assume, incur or permit or suffer to exist or to be created, assumed or
incurred, any Lien on any of the Pledged Collateral (or any interest therein),
other than Permitted Liens, and will not, without the prior written consent of
the Pledgee, sell, lease, assign, transfer or otherwise dispose of all or any
portion of the Pledged Collateral (or any interest therein).

                                      -2-
<PAGE>
 
     (b) Change of Locations, Name, Etc.  Without giving the Pledgor sixty-day's
         ------------------------------                                         
prior written notice, the Pledgor will not (i) change the Pledgor's chief
executive office, principal place of business, or the location of its books and
records relating to the Pledged Collateral or (ii) change its name, identity or
structure.

     Section 5.  Additional Shares.
                 ----------------- 

     (a) During the period this Agreement is in effect, the Pledgor shall not
permit any Issuer to issue any additional shares of capital stock or other
equity securities or interests to any Person other than the Pledgor.  Further,
the Pledgor shall not permit any Issuer to amend or modify its articles or
certificate of incorporation in a manner which would affect the voting,
liquidation, preference or other rights of a holder of the shares of stock
pledged hereunder.

     (b) The Pledgor agrees that, until this Agreement has terminated in
accordance with its terms, any additional Securities of an Issuer at any time
issued to the Pledgor or otherwise acquired by the Pledgor shall be promptly
delivered or otherwise transferred to the Pledgee as additional Pledged
Collateral and shall be subject to the Lien of, and the terms and conditions of,
this Agreement.

     Section 6.  Registration in Nominee Name, Denominations.  The Pledgee shall
                 -------------------------------------------                    
have the right (in its sole and absolute discretion) to hold the Pledged
Collateral in its own name as pledgee, the name of its nominee (as Pledgee or as
sub-agent) or the name of the applicable Pledgor, endorsed or assigned in blank
or in favor of the Pledgee.  The Pledgor will promptly give to the Pledgee
copies of any notices or other communications received by it with respect to
Pledged Collateral registered in the name of the Pledgor.  The Pledgee shall at
all times have the right to exchange the certificates representing Pledged
Collateral for certificates of smaller or larger numbers of shares for any
purpose consistent with this Agreement.

     Section 7.  Voting Rights; Dividends, etc.
                 ------------------------------

     (a)  So long as no Event of Default shall have occurred and be continuing:

          (i) the Pledgor shall be entitled to exercise any and all voting
     and/or consensual rights and powers accruing to an owner of the Pledged
     Collateral or any part thereof for any purpose not inconsistent with the
     terms and conditions of this Agreement or any agreement giving rise to or
     otherwise relating to any of the Obligations; provided, however, that the
     Pledgor shall not exercise, or refrain from exercising, any such right or
     power if any such action would have a materially adverse effect on the
     value of such Pledged Collateral in the judgment of the Pledgee;

                                      -3-
<PAGE>
 
          (ii) the Pledgor shall be entitled to retain and use any and all cash
     dividends paid on the Pledged Collateral, but any and all stock and/or
     liquidating dividends, other distributions in property, return of capital
     or other distributions made on or in respect of Pledged Collateral, whether
     resulting from a subdivision, combination or reclassification of
     outstanding Securities of an Issuer which are pledged hereunder or received
     in exchange for Pledged Collateral or any part thereof or as a result of
     any merger, consolidation, acquisition or other exchange of assets or on
     the liquidation, whether voluntary or involuntary, of an Issuer, or
     otherwise, shall be and become part of the Pledged Collateral pledged
     hereunder and, if received by the Pledgor, shall forthwith be delivered to
     the Pledgee to be held as collateral subject to the terms and conditions of
     this Agreement.

The Pledgee agrees to execute and deliver to the Pledgor, or cause to be
executed and delivered to the Pledgor, as appropriate, at the sole cost and
expense of the Pledgor, all such proxies, powers of attorney, dividend orders
and other instruments as the Pledgor may reasonably request for the purpose of
enabling the Pledgor to exercise the voting and/or consensual rights and powers
which Pledgor is entitled to exercise pursuant to clause (i) above and/or to
receive the dividends which Pledgor is authorized to retain pursuant to clause
(ii) above.

     (b) Upon the occurrence and during the continuance of an Event of Default,
all rights of the Pledgor to exercise the voting and/or consensual rights and
powers which Pledgor is entitled to exercise pursuant to subsection (a)(i) above
and/or to receive the dividends which Pledgor is authorized to receive and
retain pursuant to subsection (a)(ii) above shall cease, and all such rights
thereupon shall become immediately vested in the Pledgee, which shall have, to
the extent permitted by law, the sole and exclusive right and authority to
exercise such voting and/or consensual rights and powers which the Pledgor shall
otherwise be entitled to exercise pursuant to subsection (a)(i) above and/or to
receive and retain the dividends which the Pledgor shall otherwise be authorized
to retain pursuant to subsection (a)(ii) above.  Any and all money and other
property paid over to or received by the Pledgee pursuant to the provisions of
this subsection (b) shall be retained by the Pledgee as additional collateral
hereunder and shall be applied in accordance with the provisions of Section 9.
If the Pledgor shall receive any dividends or other property which it is not
entitled to receive under this Section, the Pledgor shall hold the same in trust
for the Pledgee, without commingling the same with other funds or property of or
held by the Pledgor, and shall promptly deliver the same to the Pledgee upon
receipt by the Pledgor in the identical form received, together with any
necessary endorsements.

      Section 8.  Remedies upon Default.
                  --------------------- 

      (a) In addition to any right or remedy that the Pledgee may have under the
Note or otherwise under Applicable Law, if an Event of Default shall have
occurred, the Pledgee may exercise any and all the rights and remedies of a
secured party under the Uniform Commercial Code as in effect in any applicable
jurisdiction and may otherwise
                                      -4-
<PAGE>
 
sell, assign, transfer, endorse and deliver the whole or, from time to time, any
part of the Pledged Collateral at a public or private sale or on any securities
exchange, for cash, upon credit or for other property, for immediate or future
delivery, and for such price or prices and on such terms as the Pledgee in its
discretion shall deem appropriate. The Pledgee shall be authorized at any sale
(if it deems it advisable to do so) to restrict the prospective bidders or
purchasers to Persons who will represent and agree that they are purchasing the
Pledged Collateral for their own account in compliance with the Securities Act
of 1933, as amended, and upon consummation of any such sale the Pledgee shall
have the right to assign, transfer, endorse and deliver to the purchaser or
purchasers thereof the Pledged Collateral so sold. Each purchaser at any sale of
Pledged Collateral shall take and hold the property sold absolutely free from
any claim or right on the part of the Pledgor, and the Pledgor hereby waives (to
the fullest extent permitted by Applicable Law) all rights of redemption, stay
and/or appraisal which the Pledgor now has or may at any time in the future have
under any Applicable Law now existing or hereafter enacted. The Pledgor agrees
that, to the extent notice of sale shall be required by Applicable Law, at least
ten days' prior written notice to the Pledgor of the time and place of any
public sale or the time after which any private sale is to be made shall
constitute reasonable notification, but notice given in any other reasonable
manner or at any other reasonable time shall constitute reasonable notification.
Such notice, in case of public sale, shall state the time and place for such
sale, and, in the case of sale on a securities exchange, shall state the
exchange on which such sale is to be made and the day on which the Pledged
Collateral, or portion thereof, will first be offered for sale at such exchange.
Any such public sale shall be held at such time or times within ordinary
business hours and at such place or places as the Pledgee may fix and shall
state in the notice or publication (if any) of such sale. At any such sale, the
Pledged Collateral, or portion thereof to be sold, may be sold in one lot as an
entirety or in separate parcels, as the Pledgee may determine in its sole and
absolute discretion. The Pledgee shall not be obligated to make any sale of the
Pledged Collateral if it shall determine not to do so regardless of the fact
that notice of sale of the Pledged Collateral may have been given. The Pledgee
may, without notice or publication, adjourn any public or private sale or cause
the same to be adjourned from time to time by announcement at the time and place
fixed for sale, and such sale may, without further notice, be made at the time
and place to which the same was so adjourned. In case the sale of all or any
part of the Pledged Collateral is made on credit or for future delivery, the
Pledged Collateral so sold may be retained by the Pledgee until the sale price
is paid by the purchaser or purchasers thereof, but the Pledgee shall not incur
any liability to the Pledgor in case any such purchaser or purchasers shall fail
to take up and pay for the Pledged Collateral so sold and, in case of any such
failure, such Pledged Collateral may be sold again upon like notice. At any
public sale made pursuant to this Agreement, the Pledgee, to the extent
permitted by Applicable Law, may bid for or purchase, free from any right of
redemption, stay and/or appraisal on the part of the Pledgor (all said rights
being also hereby waived and released to the extent permitted by Applicable
Law), any part of or all the Pledged Collateral offered for sale and may make
payment on account thereof by using any claim then due and payable to the
Pledgee from the Pledgor as a credit against the purchase price, and the Pledgee
may, upon compliance with the terms of sale and to the extent permitted by
Applicable Law, hold, retain and 

                                      -5-
<PAGE>
 
dispose of such property without further accountability to the Pledgor therefor.
For purposes hereof, a written agreement to purchase all or any part of the
Pledged Collateral shall be treated as a sale thereof; the Pledgee shall be free
to carry out such sale pursuant to such agreement and the Pledgor shall not be
entitled to the return of any Pledged Collateral subject thereto,
notwithstanding the fact that after the Pledgee shall have entered into such an
agreement all Events of Default may have been remedied or the Obligations may
have been paid in full as herein provided. The Pledgor hereby waives any right
to require any marshaling of assets and any similar right.

     (b) In addition to exercising the power of sale herein conferred upon it,
the Pledgee shall also have the option to proceed by suit or suits at law or in
equity to foreclose this Agreement and sell the Pledged Collateral or any
portion thereof pursuant to judgment or decree of a court or courts having
competent jurisdiction.

     (c) The rights and remedies of the Pledgee under this Agreement are
cumulative and not exclusive of any rights or remedies which it would otherwise
have.

     Section 9.  Application of Proceeds of Sale and Cash.  The proceeds of any
                 ----------------------------------------                      
sale of the whole or any part of the Pledged Collateral, together with any other
moneys held by the Pledgee under the provisions of this Agreement, shall be
applied by the Pledgee in the following order:

     (a) First:  to the payment of all costs and expenses incurred in connection
with such sale or other realization, including reasonable attorneys' fees
incurred if the Pledgee endeavored to collect the Obligations by or through an
attorney at law;

     (b) Second:  to the payment of the interest due upon any of the
Obligations, in any order which the Pledgee may elect;

     (c) Third:  to the payment of the principal due upon any of the Obligations
in any order which the Pledgee may elect; and

     (d) Fourth:  the balance (if any) of such proceeds shall be paid to the
Pledgor or to whomsoever may be legally entitled thereto.

The Pledgor shall remain liable and will pay, on demand, any deficiency
remaining in respect of the Obligations.

     Section 10.  Pledgee Appointed Attorney-in-Fact.  The Pledgor hereby
                  ----------------------------------                     
constitutes and appoints the Pledgee as the attorney-in-fact of the Pledgor with
full power of substitution either in the Pledgee's name or in the name of the
Pledgor to do any of the following after the occurrence of an Event of Default
and for so long as there are unsatisfied Obligations: (a) to perform any
obligation of the Pledgor hereunder in the Pledgor's name or otherwise; (b) to
ask for, demand, sue for, collect, receive, receipt and give acquittance for any
and all moneys due or to become due under and by virtue of any 

                                      -6-
<PAGE>
 
Pledged Collateral; (c) to prepare, execute, file, record or deliver notices,
assignments, financing statements, continuation statements, applications for
registration or like papers to perfect, preserve or release the Pledgee's
security interest in the Pledged Collateral or any of the documents,
instruments, certificates and agreements described in Section (b); (d) to
verify facts concerning the Pledged Collateral in its own name or a fictitious
name; (e) to endorse checks, drafts, orders and other instruments for the
payment of money payable to the Pledgor, representing any interest or dividend
or other distribution payable in respect of the Pledged Collateral or any part
thereof or on account thereof and to give full discharge for the same; (f) to
exercise all rights, powers and remedies which the Pledgor would have, but for
this Agreement, under the Pledged Collateral; and (g) to carry out the
provisions of this Agreement and to take any action and execute any instrument
which the Pledgee may deem necessary or advisable to accomplish the purposes
hereof, and to do all acts and things and execute all documents in the name of
the Pledgor or otherwise, deemed by the Pledgee as necessary, proper and
convenient in connection with the preservation, perfection or enforcement of its
rights hereunder. Nothing herein contained shall be construed as requiring or
obligating the Pledgee to make any commitment or to make any inquiry as to the
nature or sufficiency of any payment received by it, or to present or file any
claim or notice, or to take any action with respect to the Pledged Collateral or
any part thereof or the moneys due or to become due in respect thereof or any
property covered thereby, and no action taken by the Pledgee or omitted to be
taken with respect to the Pledged Collateral or any part thereof shall give rise
to any defense, counterclaim or offset in favor of the Pledgor or to any claim
or action against the Pledgee. The power or attorney granted herein is
irrevocable and coupled with an interest.

     Section 11.  Reimbursement of Pledgee.  The Pledgor agrees to pay upon
                  ------------------------                                 
demand to the Pledgee the amount of any and all reasonable expenses, including
the reasonable fees, disbursements and other charges of its counsel and of any
experts or agents, and its fully allocated internal costs, that the Pledgee may
incur in connection with (i) the custody or preservation of, or any sale of,
collection from, or other realization upon, any of the Pledged Collateral, (ii)
the exercise or enforcement of any of the rights of the Pledgee hereunder, or
(iii) the failure by the Pledgor to perform or observe any of the provisions
hereof.  Any such amounts payable as provided hereunder shall be additional
obligations secured hereby and by the other Loan Documents.

     Section 12.  Further Assurances.  The Pledgor shall, at its sole cost and
                  ------------------                                          
expense, take all action that may be necessary or desirable in the Pledgee's
sole discretion, so as at all times to maintain the validity, perfection,
enforceability and priority of the Pledgee's security interest in the Pledged
Collateral, or to enable the Pledgee to exercise or enforce its rights
hereunder, including without limitation (a) delivering to the Pledgee, endorsed
or accompanied by such instruments of assignment as the Pledgee may specify, any
and all chattel paper, instruments, letters of credit and all other advices of
guaranty and documents evidencing or forming a part of the Pledged Collateral
and (b) executing and delivering financing statements, pledges, designations,
notices and assignments, in each case in form and substance satisfactory to the
Pledgee, relating to the creation, validity, 

                                      -7-
<PAGE>
 
perfection, priority or continuation of the security interest granted hereunder.
The Pledgor agrees to take, and authorizes the Pledgee to take on the Pledgor's
behalf, any or all of the following actions with respect to any Pledged
Collateral as the Pledgee shall deem necessary to perfect the security interest
and pledge created hereby or to enable the Pledgee to enforce its rights and
remedies hereunder: (i) to register in the name of the Pledgee any Pledged
Collateral in certificated or uncertificated form; (ii) to endorse in the name
of the Pledgee any Pledged Collateral issued in certificated form; and (iii) by
book entry or otherwise, identify as belonging to the Pledgee a quantity of
securities that constitutes all or part of the Pledged Collateral registered in
the name of the Pledgee. Notwithstanding the foregoing, the Pledgor agrees that
Pledged Collateral which is not in certificated form or is otherwise in book-
entry form shall be held for the account of the Pledgee. The Pledgor hereby
authorizes the Pledgee to execute and file in all necessary and appropriate
jurisdictions (as determined by the Pledgee) one or more financing or
continuation statements (or any other document or instrument referred to in the
immediately preceding clause (b)) in the name of the Pledgor and to sign the
Pledgor's name thereto. The Pledgor authorizes the Pledgee to file any such
financing statement, document or instrument without the signature of the Pledgor
to the extent permitted by Applicable Law. To the extent permitted by Applicable
Law, a carbon, photographic, xerographic or other reproduction of this Agreement
or any financing statement is sufficient as a financing statement. Any property
comprising part of the Pledged Collateral required to be delivered to the
Pledgee pursuant to this Agreement shall be accompanied by proper instruments of
assignment duly executed by the Pledgor and by such other instruments or
documents as the Pledgee may reasonably request.

     Section 13.  Securities Act.  In view of the position of the Pledgor in
                  --------------                                            
relation to the Pledged Collateral, or because of other current or future
circumstances, a question may arise under the Securities Act of 1933, as now or
hereafter in effect, or any similar Applicable Law hereafter enacted analogous
in purpose or effect (such Act and any such similar Applicable Law as from time
to time in effect being called the "Federal Securities Laws") with respect to
any disposition of the Pledged Collateral permitted hereunder.  The Pledgor
understands that compliance with the Federal Securities Laws might very strictly
limit the course of conduct of the Pledgee if the Pledgee were to attempt to
dispose of all or any part of the Pledged Collateral in accordance with the
terms hereof, and might also limit the extent to which or the manner in which
any subsequent transferee of any Pledged Collateral could dispose of the same.
Similarly, there may be other legal restrictions or limitations affecting the
Pledgee in any attempt to dispose of all or part of the Pledged Collateral in
accordance with the terms hereof under applicable Blue Sky or other state
securities laws or similar Applicable Law analogous in purpose or effect.  The
Pledgor recognizes that in light of the foregoing restrictions and limitations
the Pledgee may, with respect to any sale of the Pledged Collateral, limit the
purchasers to those who will agree, among other things, to acquire such Pledged
Collateral for their own account, for investment, and not with a view to the
distribution or resale thereof.  The Pledgor acknowledges and agrees that in
light of the foregoing restrictions and limitations, the Pledgee, in its sole
and absolute discretion, may, in accordance with Applicable Law, (a) proceed to
make such a sale whether or not a registration statement for the purpose of

                                      -8-
<PAGE>
 
registering such Pledged Collateral or part thereof shall have been filed under
the Federal Securities Laws and (b) approach and negotiate with a single
potential purchaser to effect such sale. The Pledgor acknowledges and agrees
that any such sale might result in prices and other terms less favorable to the
seller than if such sale were a public sale without such restrictions. In the
event of any such sale, the Pledgee shall incur no responsibility or liability
for selling all or any part of the Pledged Collateral in accordance with the
terms hereof at a price that the Pledgee, in its sole and absolute discretion,
may in good faith deem reasonable under the circumstances, notwithstanding the
possibility that a substantially higher price might have been realized if the
sale were deferred until after registration as aforesaid or if more than a
single purchaser were approached. The provisions of this Section will apply
notwithstanding the existence of public or private market upon which the
quotations or sales prices may exceed substantially the price at which the
Pledgee sells.

     Section 14.  Indemnification.  The Pledgor agrees to indemnify and hold the
                  ---------------                                               
Pledgee and any corporation controlling, controlled by, or under common control
with, the Pledgee and any officer, attorney, director, shareholder, agent or
employee of the Pledgee or any such corporation (each an "Indemnified Person"),
harmless from and against any claim, loss, damage, action, cause of action,
liability, cost and expense or suit of any kind or nature whatsoever
(collectively, "Losses"), brought against or incurred by an Indemnified Person,
in any manner arising out of or, directly or indirectly, related to or connected
with this Agreement, including without limitation, the exercise by the Pledgee
of any of its rights and remedies under this Agreement or any other action taken
by the Pledgee pursuant to the terms of this Agreement; provided, however, the
Pledgor shall not be liable to an Indemnified Person for any Losses to the
extent that such Losses result from the gross negligence or willful misconduct
of such Indemnified Person.  The Pledgor's obligations under this section shall
survive the termination of this Agreement and the payment in full of the
Obligations until the third anniversary of the date of this Agreement.

     Section 15.  Continuing Security Interest.  This Agreement shall create a
                  ----------------------------                                
continuing security interest in the Pledged Collateral and shall remain in full
force and effect until it terminates in accordance with its terms.  The Pledgor
and the Pledgee hereby agree that the security interest created by this
Agreement in the Pledged Collateral shall not terminate and shall continue and
remain in full force and effect notwithstanding the transfer to the Pledgor or
any person designated by it of all or any portion of the Pledged Collateral.

     Section 16.  Security Interest Absolute.  All rights of the Pledgee
                  --------------------------                            
hereunder, the grant of a security interest in the Pledged Collateral and all
obligations of the Pledgor hereunder, shall be absolute and unconditional
irrespective of (a) any lack of validity or enforceability of the Note, any
agreement with respect to any of the Obligations or any other agreement or
instrument relating to any of the foregoing, (b) any change in the time, manner
or place of the payment of, or in any other term of, all or any of the
Obligations, or any other amendment or waiver of or any consent to any departure
from the Note, or 

                                      -9-
<PAGE>
 
any other agreement or instrument relating to any of the foregoing, or (c) any
exchange, release or nonperfection of any other collateral, or any release or
amendment or waiver of or consent to or departure from any guaranty, for all or
any of the Obligations.

     Section 17.  No Waiver.  Neither the failure on the part of the Pledgee to
                  ---------                                                    
exercise, nor the delay on its part in exercising any right, power or remedy
hereunder, nor any course of dealing between the Pledgee and the Pledgor shall
operate as a waiver thereof, nor shall any single or partial exercise of any
such right, power, or remedy hereunder preclude any other or the further
exercise thereof or the exercise of any other right, power or remedy.

     Section 18.  Notices.  Notices, requests and other communications required
                  -------                                                      
or permitted hereunder shall be in writing and shall be made by personal
delivery, telecopy or certified or registered mail, return receipt requested,
addressed as follows:

     (a)  If to the Pledgor:  Zynaxis, Inc.
                              371 Phoenixville Pike
                              Malvern, PA  19355
                              Attn:  Martyn Greenacre
                              Telephone:  (610) 889-2200
                              Telecopy:  (610) 889-2222

     (b)  If to the Pledgee:  CytRx Corporation
                              154 Technology Parkway
                              Norcross, GA  30092
                              Attn:  Jack J. Luchese
                              Telephone:  (770) 368-9500
                              Telecopy:  (770) 448-3357

or at such other address a party may specify to the other party by like notice.
All such notices and other communications shall be effective (i) if mailed, when
received; (ii) if telecopied, when transmitted; or (iii) if hand delivered, when
delivered.

     SECTION 19.  GOVERNING LAW.  THIS AGREEMENT SHALL BE GOVERNED BY, AND
                  -------------                                           
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF GEORGIA.

     Section 20.  Amendments.  No amendment or waiver of any provision of this
                  ----------                                                  
Agreement nor consent to any departure by the Pledgor 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 21.  Binding Agreement; Assignment.  This Agreement shall be
                  -----------------------------                          
binding upon and inure to the benefit of the parties hereto and their respective
successors and 

                                      -10-
<PAGE>
 
assigns, except that the Pledgor shall not be permitted to assign this Agreement
or any interest herein or in the Pledged Collateral, or any part thereof, or any
cash or property held by the Pledgee as collateral under this Agreement.

     Section 22.  Termination.  Upon indefeasible payment in full of all of the
                  -----------                                                  
Obligations, this Agreement shall terminate.  Upon termination of this Agreement
in accordance with its terms the Pledgee agrees to take such actions as the
Pledgor may reasonably request, and at the sole cost and expense of the Pledgor,
(a) to return the Pledged Collateral to the Pledgor, and (b) to evidence the
termination of this Agreement, including, without limitation, the filing of any
releases or any termination statements under the Uniform Commercial Code.

     Section 23.  Severability.  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 24.  Headings.  Section headings used herein are for convenience
                  --------                                                   
only and are not to affect the construction of or be taken into consideration in
interpreting this Agreement.

     Section 25.  Counterparts.  This Agreement may be executed in any number of
                  ------------                                                  
counterparts, each of which shall be deemed an original and all of which shall
constitute but one agreement.

     Section 26.  Definitions.  Terms not otherwise defined herein are used
                  -----------                                              
herein with the respective meanings given to them in (i) the Security Agreement
of even date herewith between Pledgor and Pledgee and (ii) the Senior Secured
Note of even date herewith executed by the Pledgor in favor of the Pledgee, as
the case may be.

     "Governmental Authority" means any national, state or local government
      ----------------------                                               
(whether domestic or foreign), any political subdivision thereof or any other
governmental, quasi-governmental, judicial, public or statutory instrumentality,
authority, body, agency, bureau or entity (including, without limitation, the
Federal Deposit Insurance Corporation, the Comptroller of the Currency or the
Federal Reserve Board, any central bank or any comparable authority) or any
arbitrator with authority to bind a party at law.

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<PAGE>
 
     IN WITNESS WHEREOF, the Pledgor has executed and delivered this Pledge
Agreement under seal as of this the date first written above.


                                        ZYNAXIS, INC.
                     
                     
                                        By:  /s/ Martyn Greenacre
                                            ---------------------
                                           Title: Chairman, President & CEO
                                                  -------------------------


Agreed to, accepted and acknowledged
as of the date first written above.


CYTRX CORPORATION


By:  /s/ Jack Luchese
     ----------------
  Title:  Chairman, President & CEO
          -------------------------

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