1


                                                                     Exhibit 5.1

                                KIRKLAND & ELLIS
                PARTNERSHIPS INCLUDING PROFESSIONAL CORPORATIONS

                                 Citicorp Center
                              153 East 53rd Street
                          New York, New York 10022-4675

                                  212 446-4800
                                                                      Facsimile:
                                                                    212 446-4900



                                  June 11, 1998


MCMS, Inc.
16399 Franklin Road
Nampa, Idaho 83687

         Re:      Offer by MCMS, Inc. to Exchange its (i) Series B 9 3/4% Senior
                  Subordinated Notes due 2008 for any and all of its 9 3/4%
                  Senior Subordinated Notes due 2008, (ii) Series B Floating
                  Interest Rate Subordinated Term Securities due 2008 for any
                  and all of its Floating Interest Rate Subordinated Term
                  Securities due 2008 and (iii) Series B 12 1/2% Senior
                  Exchangeable Preferred Stock for any and all of its 12 1/2%
                  Senior Exchangeable Preferred Stock

Ladies and Gentlemen:

         We are acting as special counsel to MCMS, Inc., an Idaho corporation
(the "Company"), in connection with the proposed registration by the Company of
(i) up to $145,000,000 in aggregate principal amount of the Company's Series B 9
3/4% Senior Subordinated Notes due 2008 (the "Fixed Rate Exchange Notes"), (ii)
up to $30,000,000 in aggregate principal amount of the Company's Series B
Floating Interest Rate Subordinated Term Securities due 2008 (the "Floating Rate
Exchange Notes" and, together with the Fixed Rate Exchange Notes, the "Exchange
Notes"), (iii) up to $47,500,000 of the Company's Series B 12 1/2% Senior
Exchangeable Preferred Stock (the "Exchange Preferred Stock" and together with
the Exchange Notes, the "Exchange Securities") and (iv) $47,500,000 of 12 1/2%
Subordinated Exchange Debentures due 2010 (the "Exchange Debentures"), pursuant
to a Registration Statement on Form S-4 originally filed with the Securities and
Exchange Commission (the "Commission") on April 24, 1998 under the Securities
Act of 1933, as amended (the "Securities Act") (such Registration Statement, as
amended or supplemented, is hereinafter referred to as the "Registration
Statement"), for the purpose of effecting an exchange offer (the "Exchange
Offer") for 
   2
   
(i) the Company's 9 3/4% Senior Subordinated Notes due 2008 (the "Fixed Rate
Notes"), (ii) the Company's Floating Interest Rate Subordinated Term Securities
due 2008 (the "Floating Rate Notes" and, together with the Fixed Rate Notes, the
"Notes") and (iii) the Company's 12 1/2% Senior Exchangeable Preferred Stock
(the "Preferred Stock" and together with the Notes, the "Securities"),
respectively. The Fixed Rate Exchange Notes are to be issued pursuant to the
Indenture (the "Indenture"), dated as of February 26, 1998, between the Company
and United State Trust Company of New York, as Trustee (the "Trustee"), in
exchange for and in replacement of the Company's outstanding Fixed Rate Notes,
of which $145,000,000 in aggregate principal amount is outstanding. The Floating
Rate Notes are to be issued pursuant to the Indenture, in exchange for and in
replacement of the Company's outstanding Floating Rate Notes, of which
$30,000,000 in aggregate principal amount is outstanding. The Exchange Preferred
Stock are to be issued pursuant to the Certificate of Designation (the
"Certificate of Designation), dated as of February 24, 1998, in exchange for and
in replacement of the Company's outstanding Preferred Stock, of which
$25,000,000 in liquidation preference is outstanding.
    

         In connection with the Exchange Offer, we have examined originals, or
copies certified or otherwise identified to our satisfaction, of such documents,
corporate records and other instruments as we have deemed necessary for the
purposes of this opinion, including (i) the corporate and organizational
documents of the Company, (ii) minutes and records of the corporate proceedings
of the Company with respect to the issuance of the Exchange Securities, (iii)
the Registration Statement and exhibits thereto and (iv) the Registration Rights
Agreement, dated as of February 26, 1998, by and between the Company and BT
Alex. Brown.

         For purposes of this opinion, we have assumed the authenticity of all
documents submitted to us as originals, the conformity to the originals of all
documents submitted to us as copies and the authenticity of the originals of all
documents submitted to us as copies. We have also assumed the genuineness of the
signatures of persons signing all documents in connection with which this
opinion is rendered, the authority of such persons signing on behalf of the
parties thereto other than the Company, and the due authorization, execution and
delivery of all documents by the parties thereto other than the Company. 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.

         Based upon and subject to the foregoing qualifications, assumptions and
limitations and the further limitations set forth below, we are of the opinion
that:

         (1) The Company is a corporation existing and in good standing under
the General Corporation Law of the State of Idaho.
   3

   
         (2) When, as and if (i) the Registration Statement shall have become
effective pursuant to the provisions of the Securities Act, (ii) the Indenture
shall have been qualified pursuant to the provisions of the Trust Indenture Act
of 1939, as amended,(iii) the Exchange Indenture shall have been qualified 
pursuant to the provisions of the Trust Indenture Act of 1939, as amended, (iv)
the Securities shall have been validly tendered to the Company, (v) the Exchange
Securities shall have been issued in the form and containing the terms described
in the Registration Statement, the Indenture, the Certificate of Designation and
the resolutions of the Company's Board of Directors (or authorized committee
thereof) authorizing the foregoing, the Exchange Securities when issued
pursuant to the Exchange Offer and the Exchange Debentures when issued in
exchange for the Preferred Stock or the Exchange Preferred Stock will be legally
issued and will constitute valid and binding obligations of the Company under
the terms and conditions described in the Registration Statement, the Indenture,
the Exchange Indenture, the Certificate of Designation, and any legally required
consents, approvals, authorizations and other order of the Commission and any
other regulatory authorities to be obtained.
    

   
         Our opinions expressed above are subject to the qualifications that we
express no opinion as to the applicability of, compliance with, or effect of (i)
any bankruptcy, insolvency, reorganization, fraudulent transfer, fraudulent
conveyance, moratorium or other similar law affecting the enforcement of
creditors' rights generally, (ii) general principles of equity (regardless of
whether enforcement is considered in a proceeding in equity or at law), (iii)
public policy considerations which may limit the rights of parties to obtain
certain remedies and (iv) other than with respect to our opinion in paragraph
1, any laws except the laws of the State of New York. We advise you that issues
addressed by this letter may be governed in whole or in part by other laws, but
we express no opinion as to whether any relevant difference exists between the
laws upon which our opinions are based and any other laws which may actually
govern. For purposes of the opinion in paragraph 1, we have relied exclusively
upon recent certificates issued by the Idaho Secretary of State with respect to
the Company, and such opinion is not intended to provide any conclusion or
assurance beyond that conveyed by such certificates. We have assumed without
investigation that there has been no relevant change or development between the
respective dates of such certificates and the date of this letter.     

         We hereby consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement. We also consent to the reference to our firm under the
heading "Legal Matters" in the Registration Statement. In giving this consent,
we do not thereby admit that we are in the category of persons whose consent is
required under Section 7 of the Securities Act of the rules and regulations of
the Commission.
   4
         We do not find it necessary for the purposes of this opinion, and
accordingly we do not purport to cover herein, the application of the securities
or "Blue Sky" laws of the various states to the issuance of the Exchange
Securities.

         This opinion is limited to the specific issues addressed herein, and no
opinion may be inferred or implied beyond that expressly stated herein. We
assume no obligation to revise or supplement this opinion should the present
laws of the State of New York be changed by legislative action, judicial
decision or otherwise.

         This opinion is furnished to you in connection with the filing of the
Registration Statement, and is not to be used, circulated, quoted or otherwise
relied upon for any other purposes.

                                            Yours very truly,

                                            /s/ Kirkland & Ellis
                                            -------------------------
                                            KIRKLAND & ELLIS