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                                                                     EXHIBIT 5.1

             [SKADDEN, ARPS, SLATE, MEAGHER & FLOM LLP LETTERHEAD]

                                                                    May 17, 2001

Regeneron Pharmaceuticals, Inc.
777 Old Saw Mill River Road
Tarrytown, New York 10591-6707

     Re: Regeneron Pharmaceuticals, Inc. Registration Statement on Form S-8

Ladies and Gentlemen:

     We have acted as special counsel to Regeneron Pharmaceuticals, Inc., a New
York corporation (the "Company"), in connection with the registration of
6,000,000 shares (the "Shares") of the Company's common stock, par value $0.001
per share (the "Common Stock"), issuable pursuant to the Company's 2000
Long-Term Incentive Plan (the "Plan").

     This opinion is being furnished in accordance with the requirements of Item
601(b)(5) of Regulation S-K under the Securities Act of 1933, as amended (the
"Act").

     In connection with this opinion, we have examined originals or copies,
certified or otherwise identified to our satisfaction, of (i) the Company's
Registration Statement on Form S-8 to be filed with the Securities and Exchange
Commission (the "Commission") on the date hereof (the "Registration Statement");
(ii) a specimen certificate evidencing the Common Stock; (iii) the Restated
Certificate of Incorporation of the Company, as presently in effect; (iv) the
Bylaws of the Company, as presently in effect; (v) the Plan; and (vi) certain
resolutions of the Board of Directors of the Company (the "Board") relating to
the Plan and the filing of the Registration Statement. We have also examined
originals or copies, certified or otherwise identified to our satisfaction, of
such records of the Company and such agreements, certificates of public
officials, certificates of officers or other representatives of the Company and
others, and such other documents, certificates and records as we have deemed
necessary or appropriate as a basis for the opinions set forth herein.

     In our examination, we have assumed the legal capacity of all natural
persons, 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 certified, conformed or photostatic copies and the
authenticity of the originals of such latter documents. In making our
examination of documents executed or to be executed by parties other than the
Company, we have assumed that such parties had or will have the power, corporate
or other, to enter into and perform all obligations thereunder and have also
assumed the due authorization by all requisite action, corporate or other, and
execution and delivery by such parties of such documents and the validity and
binding effect thereof. As to any facts material to the opinions expressed
herein which we have not independently established or verified, we have relied
upon statements and representations of officers and other representatives of the
Company and others. In rendering the opinion set forth below, we have assumed
that the certificates representing the Shares will be manually signed by one of
the authorized officers of the transfer agent and registrar for the Common Stock
and registered by such transfer agent and registrar and will conform to the
specimen thereof examined by us.

     We have also assumed that each award agreement setting forth the terms of
each grant of options or other awards under the Plan will be consistent with the
Plan and will be duly authorized and validly executed and delivered by the
parties thereto, that the grant of the awards and the price of such awards has
been approved by the Compensation Committee of the Board and that the
consideration received by the Company for the Shares delivered pursuant to the
Plan will be in an amount at least equal to the par value of such Shares.

     Members of our firm are admitted to the bar in the State of New York, and
we do not express any opinion with respect to the law of any other jurisdiction.
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     Based upon and subject to the foregoing, we are of the opinion that the
Shares have been duly authorized for issuance by the Company and, when issued
and paid for in accordance with the terms and conditions of the Plan, the Shares
will be validly issued, fully paid and nonassessable.

     In connection with rendering the opinion set forth above, we draw your
attention to Section 630 of the New York Business Corporation Law (the "NYBCL"),
which may impose certain liabilities on certain shareholders of New York
corporations that have no shares listed on a national securities exchange or
regularly quoted in an over-the-counter market. Section 630 of the NYBCL does
not presently apply to the Company, and we have assumed that such section will
continue to be inapplicable to the Company.

     We hereby consent to the filing of this opinion with the Commission as an
exhibit to the Registration Statement. In giving this consent, we do not thereby
admit that we are included in the category of persons whose consent is required
under Section 7 of the Act or the rules and regulations of the Commission.

                                          Very truly yours,

                                          /s/ Skadden, Arps, Slate, Meagher &
                                          Flom LLP