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                                                                       EXHIBIT 5


               Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
                              One Financial Center
                           Boston, Massachusetts 02111



701 Pennsylvania Avenue, N.W.                               
Washington, D.C. 20004                                 Telephone: 617/542-6000
Telephone: 202/434-7300                                      Fax: 617/542-2241
Fax: 202/434-7400                                                 www.mintz.com




                                 July 10, 1997

Proteon, Inc.
Nine Technology Drive
Westborough, Massachusetts 01581



Gentlemen:

     We have acted as counsel to Proteon, Inc., a Massachusetts corporation (the
"Company"), in connection with the preparation and filing with the Securities
and Exchange Commission of a Registration Statement on Form S-8 (the
"Registration Statement"), pursuant to which the Company is registering the
issuance under the Securities Act of 1933, as amended, of a total of 800,000
shares (the "Shares") of its common stock, $.01 par value per share (the "Common
Stock"). This opinion is being rendered in connection with the filing of the
Registration Statement. All capitalized terms used herein and not otherwise
defined shall have the respective meanings given to them in the Registration
Statement.

     In connection with this opinion, we have examined the Company's Restated
Articles of Organization, as amended, and Restated By-Laws, as amended, both as
currently in effect; such other records of the corporate proceedings of the
Company and certificates of the Company's officers as we have deemed relevant;
and the Registration Statement and the exhibits thereto.

     In our examination, we have assumed the genuineness of all signatures, the
legal capacity of natural persons, the authenticity of all documents submitted
to us as originals, the conformity to original documents of all documents
submitted to us as certified or photostatic copies and the authenticity of the
originals of such copies.

      Based upon the foregoing, we are of the opinion that (i) the Shares have
been duly and validly authorized by the Company and (ii) the Shares, when sold,
will have been duly and validly issued, fully paid and non-assessable shares of
the Common Stock, free of preemptive rights.

     Our opinion is limited to Massachusetts law, and we express no opinion with
respect to the laws of any other jurisdiction. No opinion is expressed herein
with respect to the qualification of the Shares under the securities or blue sky
laws of any state or any foreign jurisdiction.


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Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.

July 10, 1997
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     We understand that you wish to file this opinion as an exhibit to the
Registration Statement, and we hereby consent thereto.

                                          Very truly yours,




                                          /s/ Mintz, Levin, Cohn, Ferris,
                                           Glovsky and Popeo, P.C.
                                          -----------------------------------
                                           Mintz, Levin, Cohn, Ferris,
                                           Glovsky and Popeo, P.C.