William J. DeLorbe, Ph.D.
                                        Executive Vice President
                                        Medical Imaging

[DuPont Logo]
Life Sciences Enterprise
DuPont Pharmaceuticals Company



October 29, 1998

H. Joseph Reiser, Ph.D.
President & CEO
Cytogen Corporation 
600 College Road East
Princeton, NJ 08540-5308

Dear Joe:

This Addendum To The Letter Agreement ("Addendum"), effective on
the date just shown above, is intended to confirm our mutual
agreement to modify the Letter Agreement between our companies
dated June 3, 1998, which Letter Agreement was modified pursuant
to a letter signed by both parties dated October 2, 1998
(together, the "Letter Agreement").  The parties hereto agree to
modify the Letter Agreement as follows:

1.   Term.  The term of the Letter Agreement shall continue until
     the earlier of December 31, 1998 or the commencement of
     marketing of Quadramet  by Berlex.

2.   Manufacture of Quadramet .  From November 1, 1998 and
     continuing for the term hereof, DuPont shall continue to
     manufacture, distribute and provide technical support for
     Quadramet  at the same level and in the same manner as it
     has since the effective date of the Letter Agreement.  As
     consideration for DuPont's efforts hereunder Cytogen shall
     pay DuPont (i) the first $205,000 of revenue received from
     sales of Quadramet  in each month and (ii) fifty percent
     (50%) of revenue from sales exceeding $410,000.  For
     example, if sales of Quadramet  during the month of November
     are $245,000, then Cytogen will pay DuPont $205,000 and
     retain $40,000.  As further example, if sales are $450,000,
     then Cytogen will pay DuPont $205,000 + $20,000 = $225,0000
     and retain $205,000 + $20,000 = $225,0000 for itself. 
     Payments under this Section 2 will be due within fifteen
     (15) days following the end of each month during the term
     hereof.

3.   Payment.  As consideration for DuPont's activities under
     Section 2(b) of the Letter Agreement, and as full accord and
     satisfaction of the payment due DuPont under Section 3(b) of
     the Letter Agreement, Cytogen shall pay DuPont four million
     dollars ($4,000,000) as follows:

(i)  three million dollars ($3,000,000) on the signing of a
     Manufacturing and Distribution Agreement by Berlex, Cytogen
     and DuPont and;

(ii) one million dollars ($1,000,000) on February 15, 1999.



<PAGE>

H. Joseph Reiser, Ph.D
October 29, 1998
Page -2-



4.   Indemnification.  Cytogen hereby agrees to indemnify and
     hold harmless DuPont, its officers, agents, and employees,
     from and against any and all loss, damage, claim, injury,
     cost or expense, including reasonable attorneys' fees and
     expenses of litigation ("Claims"), which results from any
     sale, use or disposition of Product, including, Claims
     arising in connection with patent infringement, trademark
     infringement, product liability theories, or any illness or
     personal injury, including death, or property damage.  The
     parties agree that this Article 4 shall be effective and in
     force from June 3, 1998.

If the foregoing is in accordance with your understanding, please
indicate your agreement by signing in the space provided in the
enclosed copy of this letter and promptly return it to us.

Very truly yours,

DUPONT PHARMACEUTICALS COMPANY


By: /s/ William J. DeLorbe          
    ----------------------     


Accepted and Agreed:

CYTOGEN CORPORATION


By: /s/ H. Joseph Reiser           
    --------------------