(Bench Opinion)
 NOTE: Where it is feasible, a syllabus (headnote) will be released, as is being
done in connection with this case, at the time the opinion is issued.  The
syllabus constitutes no part of the opinion of the Court but has been prepared
by the Reporter of Decisions for the convenience of the reader.  See United
                                                                     ______
States v. Detroit Lumber Co., 200 U. S. 321, 337.
______    ___________________

                       SUPREME COURT OF THE UNITED STATES

                                    Syllabus

             WARNER-JENKINSON CO., INC. v. HILTON DAVIS CHEMICAL CO.
                                       ____

    CERTIORARI TO THE UNITED STATES COURT OF APPEALS FOR THE FEDERAL CIRCUIT

          No. 95-728.   Argued October 15, 1996 - Decided March 3, 1997

Petitioner and respondent both manufacture dyes from which impurities must be
 removed.  Respondent's "'746 patent," which issued in 1985, discloses an
 improved purification process involving the "ultrafiltration" of dye through a
 porous membrane at pH levels between 6.0 and 9.0.  The inventors so limited
 their claim's pH element during patent prosecution after the patent examiner
 objected because of a perceived overlap with the earlier "Booth" patent, which
 disclosed an ultrafiltration process operating at a pH above 9.0.  In 1986,
 petitioner developed its own ultrafiltration process, which operated at a pH
 level of 5.0.  Respondent sued for infringement of the '746 patent, relying
 solely on the "doctrine of equivalents," under which a product or process that
 does not literally infringe upon the express terms of a patent claim may
 nonetheless be found to infringe if there is "equivalence" between the
 elements of the accused product or process and the claimed elements of the
 patented invention.  Graver Tank & Mfg. Co. v. Linde Air Products Co., 339
                      ______________________    _______________________
 U. S. 605, 609.  Over petitioner's objections that this is an equitable
 doctrine and is to be applied by the court, the equivalence issue was included
 among those sent to the jury, which found, inter alia, that petitioner
                                            ___________
 infringed upon the '746 patent.  The District Court, among its rulings,
 entered a permanent injunction against petitioner.  The en banc Federal
 Circuit affirmed, holding that the doctrine of equivalents continues to exist,
 that the question of equivalence is for the jury to decide, and that the jury
 had substantial evidence from which to conclude that petitioner's process was
 not substantially different from the process disclosed in the '746 patent.


















                                                                    I           

II                  WARNER-JENKINSON v. HILTON DAVIS CHEMICAL
                                    ____

                                    Syllabus


Held:
_____
   1.  The Court adheres to the doctrine of equivalents.  Pp. 4-10.
    (a) In Graver Tank, supra, at 609, the Court, inter alia, described some of
           ___________________                    ___________
 the considerations that go into applying the doctrine, such as the patent's
 context, the prior art, and the particular circumstances of the case,
 including the purpose for which an ingredient is used in the patent, the
 qualities it has when combined with the other ingredients, the function it is
 intended to perform, and whether persons reasonably skilled in the art would
 have known of the interchangeability of an ingredient not contained in the
 patent with one that was.  Pp. 4-5.
    (b) This Court rejects petitioner's primary argument, that the doctrine of
 equivalents, as set out in Graver Tank in 1950, is inconsistent with, and thus
                            ___________
 did not survive, particular aspects of Congress' 1952 revision of the Patent
 Act, 35 U. S. C. S100 et seq.  Petitioner's first three arguments in this
                       _______
 regard - that the doctrine (1) is inconsistent with S112's requirement that a
 patentee specifically "claim" the covered invention, (2) circumvents the
 patent reissue process under SS251-252, and (3) is inconsistent with the
 primacy of the Patent and Trademark Office (PTO) in setting a patent's scope -
 were made in Graver Tank, supra, at 613-615, and n. 3, in the context of the
              ___________________
 1870 Patent Act, and failed to command a majority.  The 1952 Act is not
 materially different from the 1870 Act with regard to these matters.  Also
 unpersuasive is petitioner's fourth argument, that the doctrine of equivalents
 was implicitly rejected as a general matter by Congress' specific and limited
 inclusion of it in S112, (para.)6.  This new provision was enacted as a
 targeted cure in response to Halliburton Oil Well Cementing Co. v. Walker, 329
                              __________________________________    _______
 U. S. 1, 8, and thereby to allow so-called "means" claims describing an
 element of an invention by the result accomplished or the function served. 
 Moreover, the statutory reference to "equivalents" appears to be no more than
 a prophylactic against potential side effects of that cure, i.e., an attempt
                                                             _____
 to limit the application of the broad literal language of "means" claims to
 those means that are "equivalent" to the actual means shown in the patent
 specification.  Pp. 6-9.
    (c) The determination of equivalence should be applied as an objective
 inquiry on an element-by-element basis.  The Court is concerned that the
 doctrine, as it has come to be broadly applied since Graver Tank, conflicts
                                                      ____________
 with the Court's numerous holdings that a patent may not be enlarged beyond
 the scope of its claims.  The way to reconcile the two lines of authority is
 to apply the doctrine to each of the individual elements of a claim, rather
 than to the accused product or process as a whole.  Doing so will preserve
 some meaning for each of a claim's elements, all of which are deemed material
 to

                    WARNER-JENKINSON v. HILTON DAVIS CHEMICAL                III
                                    ____

                                    Syllabus

 defining the invention's scope.  So long as the doctrine does not encroach
 beyond these limits, or beyond related limits discussed in the Court's
 opinion, infra, at 11-15, 20, n. 8, and 21-22, it will not vitiate the central
          ______
 functions of patent claims to define the invention and to notify the public of
 the patent's scope.  Pp. 9-10.
    (d) Petitioner is correct that Graver Tank did not supersede the well-
                                   ___________
 established limitation on the doctrine of equivalents known as "prosecution
 history estoppel," whereby a surrender of subject matter during patent
 prosecution may preclude recapturing any part of that subject matter, even if
 it is equivalent to the matter expressly claimed.  But petitioner reaches too
 far in arguing that any such surrender establishes a bright line beyond which
 no equivalents may be claimed, and that the reason for an amendment during
 patent prosecution is therefore irrelevant to any subsequent estoppel.  There
 are a variety of reasons why the PTO may request a change in claim language,
 and if the patent-holder demonstrates that an amendment had a purpose
 unrelated to patentability, a court must consider that purpose in order to
 decide whether an estoppel is precluded.  Where the patent-holder is unable to
 establish such a purpose, the court should presume that the purpose behind the
 required amendment is such that prosecution history estoppel would apply. 
 Here, it is undisputed that the upper limit of 9.0 pH was added to '746 patent
 in order to distinguish the Booth patent, but the record before this Court
 does not reveal the reason for adding the lower 6.0 pH limit.  It is therefore
 impossible to tell whether the latter reason could properly avoid an estoppel. 
 Pp. 10-15. 
    (e) The Court rejects petitioner's argument that Graver Tank requires
                                                     ___________
 judicial exploration of the intent of the alleged infringer or a case's other
 equities before allowing application of the doctrine of equivalents.  Although
 Graver Tank certainly leaves room for the inclusion of intent-based elements
 ___________
 in the doctrine, the Court does not read the case as requiring proof of
 intent.  The better view, and the one consistent with Graver Tank's
                                                       ___________
 predecessors, see e.g., Winans v. Denmead, 15 How. 330, 343, and the objective
                   ____________    ________
 approach to infringement, is that intent plays no role in the doctrine's
 application.  Pp. 15-18. 
    (f) The Court also rejects petitioner's proposal that in order to minimize
 conflict with the notice function of patent claims, the doctrine of
 equivalents should be limited to equivalents that are disclosed within the
 patent itself.  Insofar as the question under the doctrine is whether an
 accused element is equivalent to a claimed element, the proper time for
 evaluating equivalency - and knowledge of interchangeability between elements -
  is at the time of infringement, not at the time the patent was issued. 
 P. 18.

IV                  WARNER-JENKINSON v. HILTON DAVIS CHEMICAL
                                    ____

                                    Syllabus

    (g) The Court declines to consider whether application of the doctrine of
 equivalents is a task for the judge or for the jury, since resolution of that
 question is not necessary to answer the question here presented.  Pp. 19-20.
    (h) In the Court's view, the particular linguistic framework used to
 determine "equivalence," whether the so-called "triple identity" test or the
 "insubstantial differences" test, is less important than whether the test is
 probative of the essential inquiry: Does the accused product or process
 contain elements identical or equivalent to each claimed element of the
 patented invention?  Different linguistic frameworks may be more suitable to
 different cases, depending on their particular facts.  The Court leaves it to
 the Federal Circuit's sound judgment in this area of its special expertise to
 refine the formulation of the test for equivalence in the orderly course of
 case-by-case determinations.  Pp. 21-22.
   2.  Because the Federal Circuit did not consider all of the requirements of
 the doctrine of equivalents as described by the Court in this case,
 particularly as related to prosecution history estoppel and the preservation
 of some meaning for each element in a claim, further proceedings are
 necessary.  P. 22.

62 F. 3d 1512, reversed and remanded.

 THOMAS, J., delivered the opinion for unamimous Court.  GINSBURG, J., filed a
concurring opinion, in which KENNEDY, J., joined.