Exhibit 5.01

                          [SIDLEY & AUSTIN LETTERHEAD]

                                 March 29, 1999

Merrill Lynch Investment Partners Inc., 
  as general partner of ML Principal Protection L.P.
Princeton Corporate Campus
800 Scudders Mill Road
Section 2G
Plainsboro, New Jersey 08536

            RE:  ML PRINCIPAL PROTECTION L.P.
                 2,500,000 UNITS OF LIMITED PARTNERSHIP INTEREST (THE "UNITS")

Dear Sir or Madam:

      We refer to Post-Effective Amendment No. 1 to the Registration Statement 
on Form S-1 filed by ML Principal Protection L.P., a Delaware limited
partnership (the "Partnership"), and ML Principal Protection Trading L.P., a
Delaware limited partnership (the "Trading Partnership") with the Securities and
Exchange Commission under the Securities Act of 1933, as amended (the
"Securities Act"), on or about March 30, 1999 (the "Registration Statement;"
Registration No. 333-60567). Capitalized terms not defined herein have the
meanings specified in the Registration Statement.

            We are familiar with the proceedings to date with respect to the
proposed issuance and sale of the Units pursuant to the Prospectus and have
examined such records, documents and questions of law, and satisfied ourselves
as to such matters of fact, as we have considered relevant and necessary as a
basis for this opinion.

            For purposes of rendering this opinion, 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
originals of all documents submitted to us as certified or photostatic copies
and the authenticity of the original of such copies.

            Based on the foregoing, we are of the opinion that:

            1. The Partnership and the Trading Partnership have each been duly
formed and are validly existing in good standing as limited partnerships under
the Delaware Revised Uniform Limited Partnership Act (the "Act").

            2. The General Partner has taken all necessary corporate action
required to be taken by it to authorize the issuance and sale of the Units to
the Limited Partners and to authorize the admission to the Partnership of the
Limited Partners as limited partners of the Partnership.

            3. Assuming (i) the due authorization, execution and delivery to the
General Partner of a Subscription Agreement by each subscriber for Units (the
"Subscribers"), (ii) the due acceptance by the General Partner of each
Subscription Agreement and the due acceptance by the General Partner of the
admission of each of the Subscribers as limited partners of the Partnership,
(iii) the payment by each Subscriber of the full consideration due for 


SIDLEY & AUSTIN                                                          CHICAGO

Merrill Lynch Investment Partners Inc.
March 29, 1999
Page 2               


the Units to which it subscribed, (iv) that the books and records of the
Partnership set forth all information required by the Limited Partnership
Agreement and the Act, including all information with respect to all persons and
entities to be admitted as Partners and their contributions to the Partnership,
(v) that the Subscribers, as limited partners of the Partnership, do not
participate in the control of the business of the Partnership within the meaning
of the Act, (vi) that the Units are offered and sold as described in the
Prospectus and the Limited Partnership Agreement and (vii) that the Subscribers
meet all of the applicable suitability standards set forth in the Prospectus and
that the representations and warranties of the Subscribers in their respective
Subscription Agreements are true and correct, the Units to be issued to the
Subscribers will represent valid and legally issued limited partner interests in
the Partnership and will be fully paid and nonassessable limited partner
interests in the Partnership, as to which the Subscribers, as limited partners
of the Partnership, will have no liability in excess of their obligations to
make contributions to the Partnership, their obligations to make other payments
provided for in the Limited Partnership Agreement and their share of the
Partnership's assets and undistributed profits (subject to the obligation of a
Limited Partner to repay funds distributed to such Limited Partner by the
Partnership in certain circumstances).

            4. There are no provisions in the Limited Partnership Agreement the
inclusion of which, subject to the terms and conditions therein, would cause the
Limited Partners, as limited partners of the Partnership, to be deemed to be
participating in the control of the business of the Partnership within the
meaning of the Act.

            This opinion is limited to the Act and the General Corporation Law
of the State of Delaware. We express no opinion as to the application of the
securities or blue sky laws of the various states (including the State of
Delaware) to the sale of the Units.

            We hereby consent to the filing of this opinion as an Exhibit to the
Registration Statement and to all references to our firm included in or made a
part of the Registration Statement.

                                               Very truly yours,


                                               SIDLEY & AUSTIN