EXHIBIT 5.1

                                HALE AND DORR LLP
                                COUNSELORS AT LAW
                   650 COLLEGE ROAD EAST, PRINCETON, NJ 08540
                         609-750-7600 o FAX 609-750-7700


                                           September 29, 2003

Cytogen Corporation
650 College Road East, CN 5308, Suite 3100
Princeton, New Jersey 08540

Registration Statement on Form S-3

Ladies and Gentlemen:

         This  opinion is  furnished to you in  connection  with a  Registration
Statement  on Form  S-3 (the  "Registration  Statement")  to be  filed  with the
Securities and Exchange  Commission (the "Commission")  under the Securities Act
of 1933, as amended (the "Securities Act"), for the registration of an aggregate
of 1,172,332  issued  shares (the "Issued  Shares") of Common  Stock,  $0.01 par
value  per share  (the  "Common  Stock"),  of  Cytogen  Corporation  a  Delaware
corporation (the  "Company"),  and 1,522,332 shares of Common Stock to be issued
sold and delivered  against payment therefor in accordance with certain warrants
to purchase  Common Stock (the  "Warrants")  issued by the Company (the "Warrant
Shares",  and  collectively  with the Issued Shares,  the "Shares").  All of the
Shares are being  registered  on behalf of certain  stockholders  of the Company
(the "Selling Stockholders").

         We are  acting  as  counsel  for the  Company  in  connection  with the
registration  for resale of the Shares.  We have  examined  signed copies of the
Registration  Statement to be filed with the  Commission.  We have also examined
and  relied  upon  minutes  of  meetings  of the  stockholders  and the Board of
Directors of the Company as provided to us by the Company, stock record books of
the Company as provided to us by the Company,  the Certificate of  Incorporation
and Bylaws of the Company,  each as restated  and/or  amended to date,  and such
other  documents  as we have deemed  necessary  for  purposes of  rendering  the
opinions hereinafter set forth.

         In our  examination  of the  foregoing  documents,  we have assumed the
genuineness of all signatures, the authenticity of all documents submitted to us
as originals, the conformity to original documents of all documents submitted to
us as copies, the authenticity of the originals of such latter documents and the
legal competence of all signatories to such documents.

         Our opinion below,  insofar as it relates to the Selling  Stockholders'
shares  being fully paid,  is based  solely on a  certificate  of the  Principal
Financial and Accounting Officer of the Company confirming the Company's receipt
of the consideration  called for by the applicable  resolutions  authorizing the
issuance of such shares.

         We assume that the appropriate action will be taken, prior to the offer
and sale of the Shares,  to  register  and qualify the Shares for sale under all
applicable state securities or "blue sky" laws.

         We  express  no  opinion  herein  as  to  the  laws  of  any  state  or
jurisdiction  other than the state laws of the State of New Jersey,  the General
Corporation  Law of the State of  Delaware  and the  federal  laws of the United
States of America.

         Based upon and subject to the foregoing, we are of the opinion that the
Issued Shares have been duly authorized and are validly  issued,  fully paid and
nonassessable  and that the  Warrant  Shares  when  issued,  sold and  delivered



against payment therefor in accordance with the provisions of the Warrants, will
be duly and validly issued, fully paid and non-assessable.

         It is  understood  that this  opinion is to be used only in  connection
with the offer and sale of the Shares  while the  Registration  Statement  is in
effect.

         Please note that we are opining  only as to the matters  expressly  set
forth herein,  and no opinion should be inferred as to any other  matters.  This
opinion  is based upon  currently  existing  statutes,  rules,  regulations  and
judicial  decisions,  and we disclaim any obligation to advise you of any change
in any of these sources of law or subsequent legal or factual developments which
might affect any matters or opinions set forth herein.

         We hereby  consent to the filing of this opinion with the Commission as
an exhibit to the Registration  Statement in accordance with the requirements of
Item  601(b)(5) of Regulation S-K under the Securities Act and to the use of our
name therein and in the related prospectus under the caption "Legal Matters." In
giving  such  consent,  we do not hereby  admit that we are in the  category  of
persons whose consent is required  under Section 7 of the  Securities Act or the
rules and regulations of the Commission.

                                           Very truly yours,

                                           /s/ HALE AND DORR LLP