April 10, 2000

Cytogen Corporation
600 College Road East  CN 5308
Princeton, NJ  08540-5308

Ladies and Gentlemen:

The  undersigned  has  acted as  counsel  to  Cytogen  Corporation,  a  Delaware
corporation  (the  "Company"),  in connection with the preparation and filing by
the  Company  of  a  registration  statement  on  Form  S-3  (the  "Registration
Statement") including a related prospectus filed with the Registration Statement
(the  "Prospectus"),  covering the registration of up to 1,086,394 shares of the
Common Stock, $.01 par value, of the Company (the "Shares") on behalf of certain
selling stockholders.

In connection with this opinion, I have examined the Registration  Statement and
related  Prospectus,  the Company's  Restated  Certificate of Incorporation  and
Bylaws, as amended, and such other records, documents,  certificates,  memoranda
and other instruments as I have deemed necessary as a basis for this opinion.  I
have assumed the  genuineness  and  authenticity  of all  documents  reviewed as
originals,  the  conformity  to  originals of all  documents  reviewed as copies
thereof and the due execution and delivery of all documents  where due execution
and delivery are a prerequisite to the effectiveness thereof.

On the basis of the foregoing, and in reliance thereon, I am of the opinion that
the Shares are validly issued, fully paid and nonassessable.

I consent to the reference to my name under the caption  "Legal  Matters" in the
Prospectus  included  in the  Registration  Statement  and to the filing of this
opinion as an exhibit to the Registration  Statement.  In giving such consent, I
do not admit  that I come  within  the  category  of  persons  whose  consent is
required under Section 7 of the Securities Act of 1933, as amended.

Very truly yours,


/s/ Donald F. Crane, Jr.
- ------------------------
Donald F. Crane, Jr.