<PAGE>
 
                   CERTIFICATE OF THE DESIGNATIONS, POWERS,
                            PREFERENCES AND RIGHTS
                                    OF THE
             SERIES A CONVERTIBLE AND EXCHANGEABLE PREFERRED STOCK
                          (par value $.01 per share)

                                      of

                              CYTOGEN CORPORATION
                            a Delaware Corporation

                                 ____________

                        Pursuant to Section 151 of the
               General Corporation Law of the State of Delaware

                                 ____________

          The undersigned DOES HEREBY CERTIFY that the following resolution was
duly adopted by the Board of Directors of CYTOGEN CORPORATION, a Delaware
corporation (the "Corporation"), at a duly convened meeting on September 25,
1996:

          RESOLVED, that one series of the class of authorized preferred stock,
$.01 par value, of the Corporation is hereby created and that the designations,
powers, preferences and relative, participating, optional or other special
rights of the shares of such series, and qualifications, limitations or
restrictions thereof, are hereby fixed as follows:

          S1C         Definitions.

          "Board" shall mean the Board of Directors of the Corporation.
           -----                                                       

          "Business Day" shall mean any day other than Saturday, Sunday or a day
           ------------                                                         
on which federally chartered banks located in New York, New York are permitted
by law to be closed.

          "Common Stock" shall mean, collectively, the Corporation's Common
           ------------                                                    
Stock and any capital stock of any class of the Corporation (other than any
preferred stock) hereafter authorized which is not limited to a fixed amount or
percentage of par or stated value in respect of the rights of the holders
thereof to participate in dividends or in the distribution of assets upon any
liquidation, dissolution or winding up of the Corporation.

                                       1
<PAGE>
 
          "Conversion Price" shall mean, with respect to the shares of Series A
           ----------------                                                    
Preferred Stock, (a) $8.40 for the period ending on September 30, 1997, (b)
$9.00 for the period commencing on October 1, 1997 and ending on March 31, 1998,
(c) $9.80 for the period commencing on April 1, 1998 and ending on September 30,
1998, (d) $11.20 for the period commencing on October 1, 1998 and ending on
September 30, 1999, (e) $12.60 for the period commencing on October 1, 1999 and
ending on September 30, 2000 and (f) $14.00 for the period commencing on October
1, 2000 and ending on March 31, 2003; provided, in each case, that such amounts
shall be subject to adjustment as provided herein, including Section 7 hereof.

          "Conversion Stock" shall mean shares of the Corporation's Common
           ----------------                                               
Stock; provided, that if there is a modification or change which results in
securities issuable upon conversion of the Series A Preferred Stock that are
issued by an entity other than the Corporation or there is a modification or
change in the class of securities so issuable, then the term "Conversion Stock"
shall mean one share of the security issuable upon conversion of the Series A
Preferred Stock if such security is issuable in shares, or shall mean the
smallest unit in which such security is issuable if such security is not
issuable in shares.

          "Dividend Rate" shall mean, with respect to the Series A Preferred
           -------------                                                    
Stock, the same rate of dividends, if any, declared and paid by the Company from
time to time in respect of its Common Stock, assuming conversion thereof into
Common Stock as set forth in Section 3(a) hereof.

          "Exchange Right" shall mean the right, in exchange for delivery of all
           --------------                                                       
of the outstanding shares of the Series A Preferred Stock to the Corporation, to
acquire from the Corporation, 500,000 shares of Targon Common Stock through
March 31, 1998, 400,000 shares of Targon Common Stock from and after April 1,
1998 through and including September 30, 1999, 300,000 shares of Targon Common
Stock from and after October 1, 1999 through and including September 30, 2000,
and 200,000 shares of Targon Common Stock from and after October 1, 2000 through
and including September 30, 2001.

          "Excluded Stock" shall mean shares of Common Stock issued or reserved
           --------------                                                      
for issuance by the Corporation as a stock dividend payable in shares of Common
Stock, or upon any subdivision or split-up of the outstanding shares of Common
Stock, or upon conversion of shares of Series A Preferred Stock.

          "Fair Value" shall mean the fair value of any securities or assets as
           ----------                                                          
reasonably and in good faith determined by the Board.

          "Junior Securities" shall mean any of the Corporation's equity
           -----------------                                            
securities (whether or not currently authorized) other than the Series A
Preferred Stock.

          "Liquidation Value" of any share of Series A Preferred Stock as of any
           -----------------                                                    
particular date shall be equal to $5,000 per share.

                                       2
<PAGE>
 
          "Market Price" of any security shall mean the average of the closing
           ------------                                                       
prices of such security's sales on all securities exchanges on which such
security may at the time be listed, or, if there have been no sales on any such
exchange on any day, the average of the highest bid and lowest asked prices on
all such exchanges at the end of such day, or, if on any day such security is
not so listed, the average of the representative bid and asked prices quoted in
the NASDAQ System as of 4:00, New York time, or, if on any day such security is
not quoted in the NASDAQ System, the average of the highest bid and lowest asked
prices on such day in the domestic over-the-counter market as reported by the
National Quotation Bureau, Incorporated, or any similar successor organization,
in each such case averaged over a period of the 10 trading days preceding the
determination date. If at any time such security is not listed on any securities
exchange or quoted in the NASDAQ System or the over-the-counter market, the
"Market Price" shall be the Fair Value thereof.

          "Person" shall mean an individual, a partnership, a corporation, an
           ------                                                            
association, a joint stock company, a trust, a joint venture, an unincorporated
organization and a governmental entity or any department, agency or political
subdivision thereof.

          "Preferred Issuance Price" shall mean the purchase price for the
           ------------------------                                       
Series A Preferred Stock, which is $15,000 per share.

          "Series A Preferred Stock" shall have the meaning given to such term
           ------------------------                                           
in Section 2 hereof.

          "Subsidiary" shall mean any corporation of which the shares of
           ----------                                                   
outstanding capital stock possessing the voting power (under ordinary
circumstances) in electing the board of directors are, at the time as of which
any determination is being made, owned by the Corporation either directly or
indirectly through subsidiaries.

          "Targon" shall mean Targon Corporation, a Delaware corporation.
           ------                                                        

          "Targon Common Stock" shall mean shares of common stock, no par value,
           -------------------                                                  
of Targon.

          S2C         Series A Preferred Stock.

            1,000 shares of the preferred stock, $.01 par value per share, of 
the Corporation are hereby constituted as a series of preferred stock of the
Corporation designated as Series A Preferred Stock (the "Series A Preferred
Stock"). Such amount shall be adjusted by the Corporation in the event that any
adjustments to the Series A Preferred Stock are required as set forth herein,
including Section 7 hereof; the Corporation shall take all necessary or
appropriate actions and make all necessary or appropriate filings in connection
therewith.

                                       3
<PAGE>
 
          S3C         Dividends.

(a)
            General.  The holder of each share of Series A Preferred Stock,
            -------                                              
a pari passu basis with the holders of the Common Stock, as if the Series A
Preferred Stock had been converted into Common Stock immediately prior to the
record date in respect thereof, based on the Conversion Price then in effect,
shall be entitled to receive, when and as declared by the Board out of funds
legally available for the declaration and payment of dividends, cumulative cash
dividends at the Dividend Rate for such series, payable from time to time on a
pro rata basis (treating such Series A Preferred Stock as outstanding Common
Stock solely for such purpose, as described above), to the holders of record of
such shares at the start of business on each applicable record date. Except as
set forth above, such holders shall not be entitled to receive any dividends.

(b)       Payment of Dividends. If the Corporation declares a dividend, but does
          --------------------                                    
not have sufficient funds legally available for paying dividends on the Series A
Preferred Stock, the Corporation shall pay to each holder of the Series A
Preferred Stock such holder's pro rata share of the dividends accrued on such
holder's outstanding Series A Preferred Stock out of funds legally available
therefor and shall pay the remaining dividends in such installments as soon as
practicable after the Corporation has funds legally available therefor.

(c)       Distribution of Partial Dividend Payments. Except as otherwise 
          -----------------------------------------                         
herein, if at any time the Corporation pays less than the total amount of
dividends otherwise payable with respect to the Series A Preferred Stock, such
payment shall be distributed ratably among the holders of Series A Preferred
Stock based upon the aggregate dividends accrued but unpaid on the shares of
Series A Preferred Stock held by each such holder.

          S4C         Liquidation.

            Upon any liquidation, dissolution or winding up of the Corporation,
each holder of the Series A Preferred Stock, at its sole discretion, shall be
entitled to (i) receive an amount equal to the Liquidation Value per share
multiplied by the number of shares of Series A Preferred Stock held by such
holder or (ii) if all other holders of Series A Preferred Stock agree to
convert, convert all of the shares of Series A Preferred Stock then held by such
holder into shares of Common Stock, in accordance with the provisions of Section
7 hereof, effective immediately prior to the date of such liquidation, in which
case, in the case of this clause (ii), the holders of Series A Preferred Stock
shall be treated as holders of Common Stock. The Corporation shall provide
written notice of such liquidation, dissolution or winding up, not less than 30
days prior to the payment date stated therein, to each record holder of Series A
Preferred Stock.

                                       4
<PAGE>
 
          S5C         Voting Rights.

(a)         No Voting.  Subject to Section 5(b) hereof, the outstanding shares 
            ---------
of Series A Preferred Stock shall not be entitled to vote on any matter as to
which stockholders of the Corporation shall be entitled to vote. Notwithstanding
the preceding sentence, holders of Series A Preferred Stock shall be entitled to
notice of any stockholders' meeting in accordance with the By-laws of the
Corporation.

(b)         Special Voting Rights.  The Corporation shall not, without first 
            ---------------------
obtaining the affirmative vote or written consent of the holders of not less
than 50% of the then outstanding shares of Series A Preferred Stock, voting as a
single class:

(1)            amend or repeal any provision of, or add any provision to, the
     Corporation's Certificate of Incorporation or By-laws if such action would
     adversely alter or change the preferences, rights, privileges or powers of,
     or the restrictions provided for the benefit of, any shares of Series A
     Preferred Stock in this Certificate; or

(2)            increase or decrease the number of authorized shares of Series A
     Preferred Stock, except as required by Section 2 hereof.

          S6C         Exchange.

(a)         Exchange.  The holders of Series A Preferred Stock shall have the
            --------
right to exercise the Exchange Right in respect of all of the Series A Preferred
Stock (including any fraction of a share), so long as such Exchange Right is
exercised in respect of all shares of Series A Preferred Stock.

(b)         Exchange Procedure.
            ------------------ 

(1)                   Before the holders of the Series A Preferred Stock shall
     be entitled to exercise the Exchange Right (which shall be exercised on an
     all-or-nothing basis), such holder shall surrender the certificate or
     certificates, duly endorsed, at the office of the Corporation or of any
     transfer agent for the Series A Preferred Stock, and shall give written
     notice on or prior to September 30, 2001, to the Corporation at its
     principal corporate office, of the election to exercise the Exchange Right
     and shall state therein the name or names in which the certificate or
     certificates for shares of Targon Common Stock are to be issued.

(2)                   The Exchange Right shall be deemed to have been effected
     on the close of business on the date on which the certificates representing
     all of the Series A Preferred Stock to be exchanged have been surrendered
     at the principal corporate office of the Corporation. At such time as the
     Exchange Right has been exercised, the rights of 

                                       5
<PAGE>
 
     the holder of such Series A Preferred Stock as a holder shall cease, and
     the Person or Persons in whose name or names any certificate or
     certificates for shares of Targon Common Stock are to be issued by Targon,
     upon instructions by the Corporation, upon the exercise of the Exchange
     Right shall be deemed to have become the holder or holders of record of the
     shares of the Targon Common Stock represented thereby.

(3)                   As soon as possible after the exercise of the Exchange
     Right (but in any event within five business days in the case of clause (6)
     below), the Corporation shall deliver to the holder or holders exercising
     the Exchange Right:

(i)                   a certificate or certificates representing the number of
          shares of Targon Common Stock issuable by reason of such exercise in
          such name or names and such denomination or denominations as the
          holder exercising the Exchange Right has specified; and

(ii)                  payment in an amount equal to the amount payable under
          clause (6) below with respect to such exercise.

(4)                   The delivery of certificates for shares of Targon Common
     Stock upon the exercise of the Exchange Right shall be made without charge
     to the holders of such Series A Preferred Stock for any cost incurred by
     the Corporation or Targon in connection with such exercise and the related
     delivery of shares of Targon Common Stock. Upon exercise of each share of
     Series A Preferred Stock, the Corporation shall and shall cause Targon to
     take all such actions as are necessary in order to ensure that the shares
     of Targon Common Stock issuable with respect to such exercise shall be
     validly issued, fully paid and non-assessable.

(5)                   The Corporation shall not close its books against the
     transfer of Series A Preferred Stock upon exercise of the Exchange Right in
     any manner which interferes with the timely exercise of the Exchange Right.
     The Corporation shall assist and cooperate with any holder of Series A
     Preferred Stock required to make any governmental filings or obtain any
     governmental approval prior to or in connection with any exercise of the
     Exchange Right hereunder (including, without limitation, making any filings
     required to be made by the Corporation).

(6)                   If any fractional interest in a share of Targon Common
     Stock would, except for the provisions of this clause (6), be deliverable
     upon any exercise of the Exchange Right, the Corporation, in lieu of
     delivering the fractional share of Targon Common Stock therefor, shall pay
     an amount to the holder thereof equal to the Market Price of such
     fractional interest as of the date of the exercise of the Exchange Right.

                                       6
<PAGE>
 
(7)                   The Corporation shall ensure that all shares of Targon
     Common Stock issued upon exercise of the Exchange Right shall, when issued,
     be duly and validly issued, fully paid and nonassessable and free from all
     taxes, liens and charges. The Corporation shall and shall cause Targon to
     take all such actions as may be necessary to ensure that all such shares of
     Targon Common Stock may be so issued without violation of any applicable
     law or governmental regulation or any requirements of any domestic
     securities exchange or market upon which shares of Targon Common Stock may
     be listed (except for official notice of issuance which shall be
     immediately delivered by Targon upon each such issuance and except for
     filings, notices of applicability and permissions solely within the control
     of, or laws and regulations solely applicable to, the holders of Series A
     Preferred Stock).

(8)                   If the holders of the Series A Preferred Stock shall not
     have exercised the Exchange Right on or prior to March 31, 2001, such
     Exchange Right shall lapse.

          S7C         Conversion.

(a)         Conversion.  All holders of Series A Preferred Stock shall have 
            ----------
the right to convert all of the Series A Preferred Stock (including any fraction
of a share), so long as all shares of Series A Preferred Stock are converted at
the same time. The number of shares of Conversion Stock issuable upon such
conversion shall be computed by (A) multiplying the number of shares of Series A
Preferred Stock to be converted by the Preferred Issuance Price for the Series A
Preferred Stock and (B) dividing the result of the calculation in clause (A)
above by the Conversion Price for the Series A Preferred Stock then in effect.
If any holder of Series A Preferred Stock has exercised its Exchange Right, then
the conversion right set forth in this Section 7 with respect to all remaining
shares of Series A Preferred Stock shall expire.

(b)         Conversion Procedure.
            -------------------- 

(1)                   Before any holder of shares of the Series A Preferred
     Stock shall be entitled to convert any of such shares into shares of Common
     Stock, such holder shall surrender the certificate or certificates, duly
     endorsed, at the office of the Corporation or of any transfer agent for the
     Series A Preferred Stock, and shall give written notice, to the Corporation
     at its principal corporate office, of the election to convert such shares
     and shall state therein the name or names in which the certificate or
     certificates for shares of Common Stock are to be issued.

(2)                   Each conversion of the Series A Preferred Stock shall be
     deemed to have been effected on the close of business on the date on which
     the certificate or certificates representing the Series A Preferred Stock
     to be converted have been 

                                       7
<PAGE>
 
     surrendered at the principal corporate office of the Corporation. At such
     time as such conversion has been effected, the rights of the holder of such
     Series A Preferred Stock as a holder shall cease, and the Person or Persons
     in whose name or names any certificate or certificates for shares of
     Conversion Stock are to be issued upon such conversion shall be deemed to
     have become the holder or holders of record of the shares of Conversion
     Stock represented thereby.

(3)                   As soon as possible after a conversion has been effected
     (but in any event within five business days in the case of clause (6)
     below), the Corporation shall deliver to the converting holder:

(i)                   a certificate or certificates representing the number of
          shares of Conversion Stock issuable by reason of such conversion in
          such name or names and such denomination or denominations as the
          converting holder has specified; and

(ii)                  payment in an amount equal to the amount payable under
          clause (6) below with respect to such conversion.

(4)                   The issuance of certificates for shares of Conversion
     Stock upon conversion of the Series A Preferred Stock shall be made without
     charge to the holders of such Series A Preferred Stock for any cost
     incurred by the Corporation in connection with such conversion and the
     related issuance of shares of Conversion Stock. Upon conversion of each
     share of Series A Preferred Stock, the Corporation shall take all such
     actions as are necessary in order to ensure that the Conversion Stock
     issuable with respect to such conversion shall be validly issued, fully
     paid and nonassessable.

(5)                   The Corporation shall not close its books against the
     transfer of Series A Preferred Stock or of Conversion Stock issued or
     issuable upon conversion of Series A Preferred Stock in any manner which
     interferes with the timely conversion of Series A Preferred Stock. The
     Corporation shall assist and cooperate with any holder of Series A
     Preferred Stock or Conversion Stock required to make any governmental
     filings or obtain any governmental approval prior to or in connection with
     any conversion of shares hereunder (including, without limitation, making
     any filings required to be made by the Corporation).

(6)                   If any fractional interest in a share of Conversion Stock
     would, except for the provisions of this clause (6), be deliverable upon
     any conversion of the Series A Preferred Stock, the Corporation, in lieu of
     delivering the fractional share therefor, shall pay an amount to the holder
     thereof equal to the Market Price of such fractional interest as of the
     date of conversion.

                                       8
<PAGE>
 
(7)                   The Corporation shall at all times reserve and keep
     available out of its authorized but unissued shares of Conversion Stock,
     solely for the purpose of issuance upon the conversion of the Series A
     Preferred Stock, such number of shares of Conversion Stock issuable upon
     the conversion of all outstanding Series A Preferred Stock. All shares of
     Conversion Stock which are so issuable shall, when issued, be duly and
     validly issued, fully paid and nonassessable and free from all taxes, liens
     and charges. The Corporation shall take all such actions as may be
     necessary to ensure that all such shares of Conversion Stock may be so
     issued without violation of any applicable law or governmental regulation
     or any requirements of any domestic securities exchange or market upon
     which shares of Conversion Stock may be listed (except for official notice
     of issuance which shall be immediately delivered by the Corporation upon
     each such issuance and except for filings, notices of applicability and
     permissions solely within the control of, or laws and regulations solely
     applicable to, the holders of Series A Preferred Stock).

(8)                   If the holders of the Series A Preferred Stock shall not
     have exercised their right to convert or to exchange the Series A Preferred
     Stock on or prior to March 31, 2003, such Series A Preferred Stock shall be
     deemed automatically converted into Common Stock, pursuant to this Section
     7, at the Conversion Price in effect on such date; subject to adjustment as
     provided herein.

(c)       Conversion Price.
          ---------------- 

(1)                   Changes in Common Stock.  In case the Company shall at 
                      -----------------------
     any time or from time to time after the date of this Agreement (i) pay a
     dividend or make any other distribution with respect to its Common Stock in
     shares of Common Stock, (ii) subdivide its outstanding shares of Common
     Stock into a larger number of shares of Common Stock, (iii) combine its
     outstanding shares of Common Stock or (iv) issue any shares of its capital
     stock or other assets in a reclassification or reorganization of the Common
     Stock (including any such reclassification in connection with a
     consolidation or merger in which the Company is the continuing entity),
     then the number and kind of shares of capital stock of the Company or other
     assets that may be received upon the conversion of the Series A Preferred
     Stock shall be adjusted to the number of shares of Conversion Stock and
     amount of any such securities, cash or other property of the Company which
     the holders would have owned or have been entitled to receive after the
     happening of any of the events described above had the Series A Preferred
     Stock been converted immediately prior to the record date (or, if there is
     no record date, the effective date) for such event. An adjustment made
     pursuant to this clause (1) shall become effective retroactively
     immediately after the effective date in other cases. Any Conversion Stock
     or other assets to be acquired as a result of such adjustment shall not be
     issued prior to the effective date of such event. For the purposes of this
     clause (1), the 

                                       9
<PAGE>
 
     number of shares of Common Stock at any time outstanding shall not include
     shares held in the treasury of the Company.

(2)                   Issuance of Rights. In case the Company shall issue, at
                      ------------------
     any time on or prior to March 31, 1998, to all holders of its Common Stock
     rights, options or warrants to subscribe for or purchase, or other
     securities exchangeable for or convertible into, shares of Common Stock
     (any such rights, options, warrants or other securities, collectively,
     "Rights") (excluding rights to purchase Common Stock pursuant to a Company
     plan for reinvestment of dividends or interest) at a subscription offering,
     exercise or conversion price per share (as defined below, the "offering
     price per share") which, before deduction of customary discounts and
     commissions, is lower than the current Market Price per share of Common
     Stock on the record date of such issuance of grant, whether or not, in the
     case of Rights, such Rights are immediately exercisable or convertible,
     then the number of shares of Conversion Stock issuable upon conversion of
     the Series A Preferred Stock shall be adjusted by multiplying the number of
     shares of Conversion Stock issuable upon conversion of the Series A
     Preferred Stock immediately prior to any adjustment in connection with such
     issuance or grant by a fraction, the numerator of which shall be the number
     of shares of Common Stock outstanding (exclusive of any treasury shares) on
     the record date of issuance or grant of such Rights plus the number of
     additional shares of capital stock that would be issued upon exercise of
     the Rights, and the denominator of which shall be the number of shares of
     Common Stock outstanding (exclusive of any treasury shares) on the record
     date of issuance or grant of such Rights plus the number of shares which
     the aggregate offering price (as defined below) of the total number of
     shares of Common Stock so offered would purchase at the current Market
     Price per share of Common Stock on the record date, less customary
     discounts and commissions. Such adjustment shall be made immediately after
     the record date for the issuance or granting of such Rights. For purposes
     of this clause , the "offering price per share" of Common Stock shall in
     the case of Rights be determined by dividing (x) the total amount received
     or receivable by the Company in consideration of the issuance of such
     Rights plus the total consideration payable to the Company upon exercise
     thereof (the "aggregate offering price"), by (y) the total number of shares
     of Common Stock covered by such Rights. In case the Company shall issue, at
     any time on or after April 1, 1998, to all holders of its Common Stock any
     Rights (excluding rights to purchase Common Stock pursuant to a Company
     plan for reinvestment of dividends or interest), at the offering price per
     share which, before deduction of customary discounts and commissions, is
     lower than the current Market Price per share of Common Stock on the record
     date of such issuance or grant, whether or not, in the case of Rights, such
     Rights are immediately exercisable or convertible, then the holders of the
     Series A Preferred Stock shall be entitled to receive all of the same
     Rights pro rata as the holders of Common Stock, on an as-converted basis,
     as and when distributed to the holders of Common Stock. For the purposes of
     this clause (2), the number of shares of Common

                                       10
<PAGE>
 
Stock at any time outstanding shall not include shares held in the treasury of 
the Company. The Company shall not issue any Rights in respect of shares of 
Common Stock held in the treasury of the Company.

        (3)  Dividends and Distributions. In case the Company shall distribute 
             ---------------------------
to all holders of its shares of Common Stock any dividend or other distribution 
of evidences of its indebtedness or other assets (in each case other than cash 
dividends and other than as provided in clause (1) above in which the holders of
the Series A Preferred Stock are otherwise entitled to share, as provided 
herein) or Rights, then, in each case, all holders of the Series A Preferred 
Stock shall be entitled to receive all of the same dividends, distributions or 
Rights, as the case may be, as the holders of Common Stock, on an as-converted 
basis, as and when distributed to the holders of Common Stock, at such time, if 
any, that the holders of the Series A Preferred Stock shall have elected to 
convert such stock to Common Stock, as provided herein.

        (4)  Computations. For the purpose of any computation under clauses (1)
             ------------
and (2) above, the current Market Price per share of Common Stock at any date
shall be as set forth in (i) the definition of Market Value for the 10
consecutive trading days commencing 20 trading days prior to the earlier to
occur of (A) the date as of which the market price is to be computed or (B) the
last full trading day before the commencement of "ex-dividend" trading in the
Common Stock relating to the event giving rise to the adjustment required by
clause (1) or (2) or (ii) any other arm's-length adjustment formula that the
Board may use in good faith. In the event the Common Stock of the Company is not
then publicly traded or if for any other reason the current market price per
share cannot be determined pursuant to the foregoing provisions of this clause
(4) the current market price per share shall be the Fair Value thereof.

        (5)  Adjustment. Whenever the number of shares of Conversion Stock 
             ---------- 
issuable upon exercise of the Series A Preferred Stock is adjusted as provided
under clause (1) or (2), the Conversion Price payable upon conversion of the
Series A Preferred Stock shall be adjusted by multiplying such Conversion Price
immediately prior to such adjustment by a fraction, the numerator of which shall
be the number of shares of Conversion Stock issuable upon conversion of any
shares of Series A Preferred Stock immediately prior to such adjustment, and the
denominator of which shall be the number of shares of Conversion Stock issuable
upon conversion of any shares of Series A Preferred Stock immediately
thereafter.

        (6)  Securities. For the purpose of this Section (7), the term "shares 
             ----------
of Common Stock" shall mean (i) the class of stock designated as Common Stock,
par value $.01 per share, of the Company on the date of this Agreement or (ii)
any other class of stock resulting from successive changes or reclassifications
of such shares consisting



                                      11
<PAGE>
 
     solely of changes in par value, or from par value to no par value, or from
     no par value to par value.

(7)            Re-Adjustment.  If, at any time after any adjustment to the 
               -------------   
number of Shares of Conversion Stock issuable upon conversion of the Series A
Preferred Stock or the Conversion price shall have been made pursuant to
clause (5) of this Section 6, any rights, options, warrants or other securities
convertible into or exchangeable for shares of Common Stock shall have expired,
or any thereof shall not have been exercised, the Conversion Price and the
number of shares of Conversion Stock issuable upon conversion of the Preferred
Stock shall, upon such expiration, be readjusted and shall thereafter be such as
it would have been had it been originally adjusted (or had the original
adjustment not been required, as the case may be) as if (A) the only shares of
Common Stock offered were the shares of Common Stock, if any, actually issued or
sold upon the exercise of such rights, options or warrants and (B) such shares
of Common Stock, if any, were issued or sold for the consideration actually
received by the Company upon such exercise plus the aggregate consideration, if
any, actually received by the Company for the issuance, sale or grant of all
such rights, options or warrants whether or not exercised; provided, further
that no such readjustment shall have the effect of increasing the Conversion
Price or decreasing the number of Conversion Shares issuable upon conversion of
the Series A Preferred Stock by an amount (calculated by adjusting such
increase or decrease as appropriate to account for all other adjustments
pursuant to this Section 6 following the date of the original adjustment
referred to above) in excess of the amount of the adjustment initially made in
respect of the issuance, sale or grant of such rights, options or warrants.

(d)       Voluntary Adjustment by the Company. The Corporation may at its
          -----------------------------------
option, at any time during the term of the Preferred Stock, reduce the then
current Conversion Price to any amount and for any period of time deemed
appropriate by the Board in its sole discretion, but subject to approval by the
holders of the majority of Series A Preferred Stock.

(e)       Reorganization, Reclassification, Consolidation, Merger or Sale.  Any
          ---------------------------------------------------------------
recapitalization, reorganization, reclassification, consolidation, merger, sale
of all or substantially all of the Corporation's assets to another Person or
other transaction which is effected in such a manner that holders of Common
Stock are entitled to receive (either directly or upon subsequent liquidation)
stock, securities or assets with respect to or in exchange for Common Stock is
referred to herein as an "Organic Change". Prior to the consummation of any
Organic Change, the Corporation shall make appropriate provisions to ensure that
each of the holders of each share of Series A Preferred Stock shall thereafter
have the right to acquire and receive, in lieu of or in addition to (as the case
may be) the shares of Conversion Stock immediately theretofore acquirable and
receivable upon the conversion of such holder's Series A Preferred Stock, such
shares of stock, securities or assets as such holder would have received in
connection with such

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Organic Change if such holder had converted its Series A Preferred Stock
immediately prior to such Organic Change. In each such case, the Corporation
shall also make appropriate provisions to ensure that the provisions of this
Section 6 hereof shall thereafter be applicable to the Series A Preferred Stock.
The Corporation shall not effect any such consolidation, merger or sale, unless
prior to the consummation thereof, the successor corporation (if other than the
Corporation) resulting from consolidation or merger or the corporation
purchasing such assets assumes by written instrument, the obligation to deliver
to each such holder such shares of stock, securities or assets as, in accordance
with the foregoing provisions, such holder may be entitled to acquire.

(f)       Notices.
          ------- 

(1)                 Immediately upon any adjustment of the Conversion Price for
     the Series A Preferred Stock, the Corporation shall give written notice 
     thereof to all holders of Series A Preferred Stock setting forth in
     reasonable detail and certifying the calculation of such adjustment.

(2)                 The Corporation shall given written notice to all holders of
     Series A Preferred Stock at least 30 days prior to the date on which the
     Corporation closes its books or takes a record for determining rights to
     receive any dividends or distributions.

(3)                 The Corporation shall also give written notice to holders of
     Series A Preferred Stock at least 30 days prior to the date on which any
     Organic Change shall take place.

          S8C       Registration of Transfer.

          The Corporation shall keep at its principal office a register for the 
registration of the record holders of the Series A Preferred Stock. Upon the 
surrender of any certificate representing Series A Preferred Stock at such 
place, the Corporation shall, at the request of the record holder of such 
certificate, execute and deliver (at the Corporation's expense, provided that 
the holder will be responsible for any transfer taxes if the certificate is 
registered in a new name) a new certificate or certificates in exchange therefor
representing in the aggregate the number of shares of the Series A Preferred 
Stock represented by the surrendered certificate. Each such new certificate 
shall be registered in such name and shall represent such number of shares of 
the Series A Preferred Stock as is requested by the holder of the surrendered 
certificate and shall be substantially identical in form to the surrendered 
certificate, and dividends shall accrue on the Series A Preferred Stock 
represented by such new certificate from the date to which dividends have been 
fully paid on such Series A Preferred Stock represented by the surrendered 
certificate.

          S9C       Replacement.

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          Upon receipt of evidence reasonably satisfactory to the Corporation 
(an affidavit of the registered holder shall be satisfactory) of the ownership 
and the loss, theft, destruction or mutilation of any certificate evidencing 
shares of the Series A Preferred Stock, and in the case of any such loss, theft 
or destruction, upon receipt of indemnity reasonably satisfactory to the 
Corporation, or, in the case of any such mutilation upon surrender of such 
certificate, the Corporation shall (at its expense) execute and deliver in lieu 
of such certificate a new certificate of like kind representing the number of 
shares of such series represented by such lost, stolen, destroyed or mutilated 
certificate, and dividends shall accrue on the Series A Preferred Stock 
represented by such new certificate from the date to which dividends have been 
fully paid on such lost, stolen, destroyed or mutilated certificate.

          S10C      Amendment and Waiver.

          No amendment, modification or waiver shall be binding or effective 
with respect to any provision of this Certificate of Designations hereof without
the prior written consent of the holders of 50% or greater of the Series A 
Preferred Stock outstanding at the time such action is taken voting as a single 
class on a Common Stock equivalent basis.

          S11C      Notices.

          Except as otherwise expressly provided hereunder, all notices referred
to herein shall be in writing and shall be delivered by registered or certified 
mail, return receipt requested and postage prepaid, or by reputable overnight 
courier or telecopy service, charges prepaid, and shall be deemed to have been 
given when so mailed or sent (a) to the Corporation, at its principal executive
offices and (b) to any stockholder, at such holder's address as it appears in
the stock records of the Corporation (unless otherwise indicated by any such
holder).

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          IN WITNESS WHEREOF, Cytogen Corporation has caused this Certificate of
Designations to be executed this 25th day of September, 1996.

                                        CYTOGEN CORPORATION



                                        By: /s/ T. Jerome Madison
                                           --------------------------------
                                         Name: T. Jerome Madison
                                         Title: Chief Financial Officer and 
                                                Secretary