NOTICE: This opinion is subject to formal revision before publication in
    the preliminary print of the United States Reports.  Readers are
    requested to notify the Reporter of Decisions, Supreme Court of the
    United States, Washington, D.C. 20543, of any typographical or other
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                       SUPREME COURT OF THE UNITED STATES

                                    --------
                                   No. 95-728
                                    --------
                      WARNER-JENKINSON COMPANY, INC., PETI-
                       TIONER v. HILTON DAVIS CHEMICAL CO.
                             ____


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

                                 [March 3, 1997]

  JUSTICE THOMAS delivered the opinion of the Court.

  Nearly 50 years ago, this Court in Graver Tank & Mfg. Co. v. Linde Air
                                     ______________________    _________
Products Co., 339 U. S. 605 (1950), set out the modern contours of what is known
____________
in patent law as the "doctrine of equivalents."  Under this doctrine, 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.  Id., at 609.  Petitioner, which was found to have infringed
                     ____
upon respondent's patent under the doctrine of equivalents, invites us to speak
the death of that doctrine.  We decline that invitation.  The significant
disagreement within the Court of Appeals for the Federal Circuit concerning the
application of Graver Tank suggests, however, that the doctrine is not free from
               ___________
confusion.  We therefore will endeavor to clarify the proper scope of the
doctrine.
                                        I

  The essential facts of this case are few.  Petitioner Warner-Jenkinson Co. and
respondent Hilton Davis Chemical Co. manufacture dyes.  Impurities in those dyes
must be removed.  Hilton Davis holds United

                                95-728 - OPINION

2                   WARNER-JENKINSON v. HILTON DAVIS CHEMICAL
                                    ____

States Patent No. 4,560,746 ('746 patent), which discloses an improved
purification process involving "ultrafiltration."  The '746 process filters
impure dye through a porous membrane at certain pressures and pH
levels, (Ftnote. 1) resulting in a high purity dye product. 
        (Ftnote. 1)
  The '746 patent issued in 1985.  As relevant to this case, the patent claims
as its invention an improvement in the ultrafiltration process as follows:

   "In a process for the purification of a dye . . . the improvement which
   comprises: subjecting an aqueous solution . . . to ultrafiltration through a
   membrane having a nominal pore diameter of 5-15 Angstroms under a hydrostatic
   pressure of approximately 200 to 400 p.s.i.g., at a pH from approximately 6.0
                                                  ______________________________
   to 9.0, to thereby cause separation of said impurities from said dye . . . ." 
   ______
   App. 36-37 (emphasis added).

The inventors added the phrase "at a pH from approximately 6.0 to 9.0" during
patent prosecution.  At a minimum, this phrase was added to distinguish a
previous patent (the "Booth" patent) that disclosed an ultrafiltration process
operating at a pH above 9.0.  The parties disagree as to why the low-end pH
limit of 6.0 was included as part of the claim. (Ftnote. 2)
                                                (Ftnote. 2)


















                    
____________________

   1)  The pH, or power (exponent) of Hydrogen, of a solution is a measure of
   1)  
its acidity or alkalinity.  A pH of 7.0 is neutral; a pH below 7.0 is acidic;
and a pH above 7.0 is alkaline.  Although measurement of pH is on a logarithmic
scale, with each whole number difference representing a ten-fold difference in
acidity, the practical significance of any such difference will often depend on
the context.  Pure water, for example, has a neutral pH of 7.0, whereas carbon-
ated water has an acidic pH of 3.0, and concentrated hydrochloric acid has a pH
approaching 0.0.  On the other end of the scale, milk of magnesia has a pH of
10.0, whereas household ammonia has a pH of 11.9.  21 Encyclopedia Americana 844
(Int'l ed. 1990).
   2)  Petitioner contends that the lower limit was added because below a pH of
   2)  
6.0 the patented process created "foaming" problems in the plant and because the
process was not shown to work below that pH level.  Brief for Petitioner 4, n.
5, 37, n. 28.  Respondent counters that the process was successfully tested to
pH levels as low as 2.2 with no effect on the process because of foaming, but
offers no particular explanation as to why the lower level of 6.0 pH was
selected.  Brief for Respondent 34, n. 34.

                                95-728 - OPINION

                    WARNER-JENKINSON v. HILTON DAVIS CHEMICAL                  3
                                    ____

  In 1986, Warner-Jenkinson developed an ultrafiltration process that operated
with membrane pore diameters assumed to be 5-15 Angstroms, at pressures of 200
to nearly 500 p.s.i.g., and at a pH of 5.0.  Warner-Jenkinson did not learn of
the '746 patent until after it had begun commercial use of its ultrafiltration
process.  Hilton Davis eventually learned of Warner-Jenkinson's use of
ultrafiltration and, in 1991, sued Warner-Jenkinson for patent infringement.
  As trial approached, Hilton Davis conceded that there was no literal
infringement, and relied solely on the doctrine of equivalents.  Over Warner-
Jenkinson's objection that the doctrine of equivalents was an equitable doctrine
to be applied by the court, the issue of equivalence was included among those
sent to the jury.  The jury found that the '746 patent was not invalid and that
Warner-Jenkinson infringed upon the patent under the doctrine of equivalents. 
The jury also found, however, that Warner-Jenkinson had not intentionally
infringed, and therefore awarded only 20% of the damages sought by Hilton Davis. 
The District Court denied Warner-Jenkinson's post-trial motions, and entered a
permanent injunction prohibiting WarnerJenkinson from practicing ultrafiltration
below 500 p.s.i.g. and below 9.01 pH.  A fractured en banc Court of Appeals for
the Federal Circuit affirmed.  62 F. 3d 1512 (CA Fed. 1995).
  The majority below held that the doctrine of equivalents continues to exist
and that its touchstone is whether substantial differences exist between the

                                95-728 - OPINION

4                   WARNER-JENKINSON v. HILTON DAVIS CHEMICAL
                                    ____

accused process and the patented process.  Id., at 1521-1522.  The court also
                                           ___
held that the question of equivalence is for the jury to decide and that the
jury in this case had substantial evidence from which it could conclude that the
Warner-Jenkinson process was not substantially different from the ultrafil-
tration process disclosed in the '746 patent.  Id., at 1525.
                                               ___
  There were three separate dissents, commanding a total of 5 of 12 judges. 
Four of the five dissenting judges viewed the doctrine of equivalents as
allowing an improper expansion of claim scope, contrary to this Court's numerous
holdings that it is the claim that defines the invention and gives notice to the
public of the limits of the patent monopoly.  Id., at 1537-1538 (Plager, J.,
                                              ___
dissenting).  The fifth dissenter, the late Judge Nies, was able to reconcile
the prohibition against enlarging the scope of claims and the doctrine of
equivalents by applying the doctrine to each element of a claim, rather than to
the accused product or process "overall."  Id., at 1574 (Nies, J., dissenting). 
                                           ___
As she explained it, "[t]he `scope' is not enlarged if courts do not go beyond
the substitution of equivalent elements."  Ibid.  All of the dissenters,
                                           _____
however, would have found that a much narrowed doctrine of equivalents may be
applied in whole or in part by the court.  Id., at 1540-1542 (Plager, J.,
                                           ___
dissenting); id., at 1579 (Nies, J., dissenting).
             ___
  We granted certiorari, 516 U. S. ___ (1996), and now reverse and remand.
                                       II
  In Graver Tank we considered the application of the doctrine of equivalents to
     ___________
an accused chemical composition for use in welding that differed from the
patented welding material by the substitution of one chemical element.  339
U. S., at 610.  The substituted element did not fall within the literal terms of
the patent claim,

                                95-728 - OPINION

                    WARNER-JENKINSON v. HILTON DAVIS CHEMICAL                  5
                                    ____

but the Court nonetheless found that the "question which thus emerges is whether
the substitution [of one element for the other] . . . is a change of such
substance as to make the doctrine of equivalents inapplicable; or conversely,
whether under the circumstances the change was so insubstantial that the trial
court's invocation of the doctrine of equivalents was justified."  Ibid.  The
                                                                   _____
Court also described some of the considerations that go into applying the
doctrine of equivalents:
     "What constitutes equivalency must be determined against the context of the
   patent, the prior art, and the particular circumstances of the case.  Equiva-
   lence, in the patent law, is not the prisoner of a formula and is not an
   absolute to be considered in a vacuum.  It does not require complete identity
   for every purpose and in every respect.  In determining equivalents, things
   equal to the same thing may not be equal to each other and, by the same
   token, things for most purposes different may sometimes be equivalents. 
   Consideration must be given to the purpose for which an ingredient is used in
   a patent, the qualities it has when combined with the other ingredients, and
   the function which it is intended to perform.  An important factor is 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."  Id., at 609.
          ___

Considering those factors, the Court viewed the difference between the chemical
element claimed in the patent and the substitute element to be "colorable only,"
and concluded that the trial court's judgment of infringement under the doctrine
of equivalents was proper.  Id., at 612.
                            ___

                                95-728 - OPINION

6                   WARNER-JENKINSON v. HILTON DAVIS CHEMICAL
                                    ____

                                        A
  Petitioner's primary argument in this Court is that the doctrine of
equivalents, as set out in Graver Tank in 1950, did not survive the 1952
                           ___________
revision of the Patent Act, 35 U. S. C. S100 et seq., because it is inconsistent
                                             _______
with several aspects of that Act.  In particular, petitioner argues: (1) the
doctrine of equivalents is inconsistent with the statutory requirement that a
patentee specifically "claim" the invention covered by a patent, 35 U. S. C.
S112; (2) the doctrine circumvents the patent reissue process - designed to
correct mistakes in drafting or the like - and avoids the express limitations on
that process, 35 U. S. C. SS251-252; (3) the doctrine is inconsistent with the
primacy of the Patent and Trademark Office (PTO) in setting the scope of a
patent through the patent prosecution process; and (4) the doctrine was
implicitly rejected as a general matter by Congress' specific and limited
inclusion of the doctrine in one section regarding "means" claiming, 35 
U. S. C. S112, (para.)6.  All but one of these arguments were made in Graver
                                                                      ______
Tank in the context of the 1870 Patent Act, and failed to command a
____
majority. (Ftnote. 3)
          (Ftnote. 3)






















                    
____________________

   3)  Graver Tank was decided over a vigorous dissent.  In that dissent,
   3)  ___________
Justice Black raised the first three of petitioner's four arguments against the
doctrine of equivalents.  See 339 U. S., at 613-614 (doctrine inconsistent with
statutory requirement to "distinctly claim" the invention); id., at 614-615
                                                            ___
(patent reissue process available to correct mistakes); id., at 615, n. 3 (duty
                                                        ___
lies with the Patent Office to examine claims and to conform them to the scope
of the invention; inventors may appeal Patent Office determinations if they
disagree with result).
 Indeed, petitioner's first argument was not new even in 1950.  Nearly 100
years before Graver Tank, this Court approved of the doctrine of equivalents in
             ___________
Winans v. Denmead, 15 How. 330 (1854).  The dissent in Winans unsuccessfully
______    _______                                      ______
argued that the majority result was inconsistent with the requirement in the
1836 Patent Act that the applicant "particularly `specify and point' out what he
claims as his invention," and that the patent protected nothing more.  Id., at
                                                                       ___
347 (Campbell, J., dissenting).

                                95-728 - OPINION

                    WARNER-JENKINSON v. HILTON DAVIS CHEMICAL                  7
                                    ____

  The 1952 Patent Act is not materially different from the 1870 Act with regard
to claiming, reissue, and the role of the PTO.  Compare, e.g., 35 U. S. C. S112
                                                         ____
("The specification shall conclude with one or more claims particularly pointing
out and distinctly claiming the subject matter which the applicant regards as
his invention") with The Consolidated Patent Act of 1870, ch. 230, S26, 16 Stat.
198, 201 (the applicant "shall particularly point out and distinctly claim the
part, improvement, or combination which he claims as his invention or discov-
ery").  Such minor differences as exist between those provisions in the 1870 and
the 1952 Acts have no bearing on the result reached in Graver Tank, and thus
                                                       ___________
provide no basis for our overruling it.  In the context of infringement, we have
already held that pre-1952 precedent survived the passage of the 1952 Act.  See
Aro Mfg. Co. v. Convertible Top Replacement Co., 365 U. S. 336, 342 (1961) (new
____________    ________________________________
section defining infringement "left intact the entire body of case law on direct
infringement").  We see no reason to reach a different result
here. (Ftnote. 4)
      (Ftnote. 4)
  Petitioner's fourth argument for an implied congres-



























                    
____________________

   4)  Petitioner argues that the evolution in patent practice from "central"
   4)  
claiming (describing the core principles of the invention) to "peripheral"
claiming (describing the outer boundaries of the invention) requires that we
treat Graver Tank as an aberration and abandon the doctrine of equivalents. 
      ___________
Brief for Petitioner 43-45.  We disagree.  The suggested change in claiming
practice predates Graver Tank, is not of statutory origin, and seems merely to
                  ___________
reflect narrower inventions in more crowded arts.  Also, judicial recognition of
so-called "pioneer" patents suggests that the abandonment of "central" claiming
may be overstated.  That a claim describing a limited improvement in a crowded
field will have a limited range of permissible equivalents does not negate the
availability of the doctrine vel non.
                             _______

                                95-728 - OPINION

8                   WARNER-JENKINSON v. HILTON DAVIS CHEMICAL
                                    ____

sional negation of the doctrine of equivalents turns on the reference to
"equivalents" in the "means" claiming provision of the 1952 Act.  Section 112,
(para.)6, a provision not contained in the 1870 Act, states: 

   "An element in a claim for a combination may be expressed as a means or step
   for performing a specified function without the recital of structure,
   material, or acts in support thereof, and such claim shall be construed to
   cover the corresponding structure, material, or acts described in the
   specification and equivalents thereof."  (Emphasis added.)
                 _______________________

Thus, under this new provision, an applicant can describe an element of his
invention by the result accomplished or the function served, rather than
describing the item or element to be used (e.g., "a means of connecting Part A
                                           ____
to Part B," rather than "a two-penny nail").  Congress enacted S112, (para.)6 in
response to Halliburton Oil Well Cementing Co. v. Walker, which rejected claims
            __________________________________    ______
that "do not describe the invention but use `conveniently functional language at
the exact point of novelty,'" 329 U. S. 1, 8 (1946) (citation omitted).  See In
                                                                             __
re Donaldson Co., 16 F. 3d 1189, 1194 (CA Fed. 1994) (Congress enacted
_________________
predecessor of S112, (para.)6 in response to Halliburton); In re Fuetterer, 319
                                             ___________   _______________
F. 2d 259, 264, n. 11 (CCPA 1963) (same); see also, 2 D. Chisum, Patents
S8.04[2], at 63-64 (1996) (discussing 1954 commentary of then-Chief Patent
Examiner P. J. Federico).  Section 112, (para.)6 now expressly allows so-called
"means" claims, with the proviso that application of the broad literal language
of such claims must be limited to only those means that are "equivalent" to the
actual means shown in the patent specification.  This is an application of the
doctrine of equivalents in a restrictive role, narrowing the application of
broad literal claim elements.  We recognized this type of role for the doctrine
of equivalents in Graver Tank itself.  339 U. S.,
                  ___________

                                95-728 - OPINION

                    WARNER-JENKINSON v. HILTON DAVIS CHEMICAL                  9
                                    ____

at 608-609.  The added provision, however, is silent on the doctrine of
equivalents as applied where there is no literal infringement.
  Because S112, (para.)6 was enacted as a targeted cure to a specific problem,
and because the reference in that provision to "equivalents" appears to be no
more than a prophylactic against potential side effects of that cure, such
limited congressional action should not be overread for negative implications. 
Congress in 1952 could easily have responded to Graver Tank as it did to the
                                                ___________
Halliburton decision.  But it did not.  Absent something more compelling than
___________
the dubious negative inference offered by petitioner, the lengthy history of the
doctrine of equivalents strongly supports adherence to our refusal in Graver
                                                                      ______
Tank to find that the Patent Act conflicts with that doctrine.  Congress can
____
legislate the doctrine of equivalents out of existence any time it chooses.  The
various policy arguments now made by both sides are thus best addressed to
Congress, not this Court.
                                        B
  We do, however, share the concern of the dissenters below that the doctrine of
equivalents, as it has come to be applied since Graver Tank, has taken on a life
                                                ___________
of its own, unbounded by the patent claims.  There can be no denying that the
doctrine of equivalents, when applied broadly, conflicts with the definitional
and public-notice functions of the statutory claiming requirement.  Judge Nies
identified one means of avoiding this conflict:
   "[A] distinction can be drawn that is not too esoteric between substitution
   of an equivalent for a component in an invention and enlarging the metes and
                                    __
   bounds of the invention beyond what is claimed.
                           ______
          .     .      .      .      .
     "Where a claim to an invention is expressed as a combination of elements,
   as here, `equivalents' in the sobriquet `Doctrine of Equivalents' refers to
   the

                                95-728 - OPINION

10                  WARNER-JENKINSON v. HILTON DAVIS CHEMICAL
                                    ____

   equivalency of an element or part of the invention with one that is
                     _______    ____
   substituted in the accused product or process.
          .     .      .      .      .
     "This view that the accused device or process must be more than
   `equivalent' overall reconciles the Supreme Court's position on infringement
                _______
   by equivalents with its concurrent statements that `the courts have no right
   to enlarge a patent beyond the scope of its claims as allowed by the Patent
   Office.'  [Citations omitted.]  The `scope' is not enlarged if courts do not
   go beyond the substitution of equivalent elements."  62 F. 3d, at 1573-1574
   (Nies, J., dissenting) (emphasis in original).

  We concur with this apt reconciliation of our two lines of precedent.  Each
element contained in a patent claim is deemed material to defining the scope of
the patented invention, and thus the doctrine of equivalents must be applied to
individual elements of the claim, not to the invention as a whole.  It is
important to ensure that the application of the doctrine, even as to an
individual element, is not allowed such broad play as to effectively eliminate
that element in its entirety.  So long as the doctrine of equivalents does not
encroach beyond the limits just described, or beyond related limits to be
discussed infra, at 11-15, 20, n. 8, and 21-22, we are confident that the
          _____
doctrine will not vitiate the central functions of the patent claims themselves.
                                       III
  Understandably reluctant to assume this Court would overrule Graver Tank,
                                                               ___________
petitioner has offered alternative arguments in favor of a more restricted
doctrine of equivalents than it feels was applied in this case.  We address each
in turn.

                                95-728 - OPINION

                    WARNER-JENKINSON v. HILTON DAVIS CHEMICAL                 11
                                    ____

                                        A
  Petitioner first argues that Graver Tank never purported to supersede a well-
                               ___________
established limit on non-literal infringement, known variously as "prosecution
history estoppel" and "file wrapper estoppel."  See Bayer Aktiengesellschaft v.
                                                    ________________________
Duphar Int'l Research B.V., 738 F. 2d 1237, 1238 (CA Fed. 1984).  According to
__________________________
petitioner, any surrender of subject matter during patent prosecution, regard-
less of the reason for such surrender, precludes recapturing any part of that
subject matter, even if it is equivalent to the matter expressly claimed. 
Because, during patent prosecution, respondent limited the pH element of its
claim to pH levels between 6.0 and 9.0, petitioner would have those limits form
bright lines beyond which no equivalents may be claimed.  Any inquiry into the
reasons for a surrender, petitioner claims, would undermine the public's right
to clear notice of the scope of the patent as embodied in the patent file.
  We can readily agree with petitioner that Graver Tank did not dispose of
                                            ___________
prosecution history estoppel as a legal limitation on the doctrine of
equivalents.  But petitioner reaches too far in arguing that the reason for an
amendment during patent prosecution is irrelevant to any subsequent estoppel. 
In each of our cases cited by petitioner and by the dissent below, prosecution
history estoppel was tied to amendments made to avoid the prior art, or
otherwise to address a specific concern - such as obviousness - that arguably
would have rendered the claimed subject matter unpatentable.  Thus, in Exhibit
                                                                       _______
Supply Co. v. Ace Patents Corp., Chief Justice Stone distinguished inclusion of
__________    _________________
a limiting phrase in an original patent claim from the "very different"
situation in which "the applicant, in order to meet objections in the Patent
Office, based on references to the prior art, adopted the phrase as a substitute
        ____________________________________
for the

                                95-728 - OPINION

12                  WARNER-JENKINSON v. HILTON DAVIS CHEMICAL
                                    ____

broader one" previously used.  315 U. S. 126, 136 (1942) (emphasis added). 
Similarly, in Keystone Driller Co. v. Northwest Engineering Corp., 294 U. S. 42
              ____________________    ____________________________
(1935), estoppel was applied where the initial claims were "rejected on the
prior art," id., at 48, n. 6, and where the allegedly infringing equivalent
            ___
element was outside of the revised claims and within the prior art that formed
the basis for the rejection of the earlier claims, id., at
                                                   ___
48. (Ftnote. 5)
    (Ftnote. 5)
  It is telling that in each case this Court probed the reasoning behind the
Patent Office's insistence upon a change in the claims.  In each instance, a
change was demanded because the claim as otherwise written was viewed as not
describing a patentable invention at all - typically because what it described
was encompassed within the prior art.  But, as the United States informs us,
there are a variety of other reasons why the PTO may request a change in claim
language.  Brief for United States as Amicus Curiae 22-23 (counsel for the PTO
                                      _____________
also appearing on the brief).  And if the PTO has been requesting changes in
claim language without the intent to limit equivalents or, indeed, with the
expecta-























                    
____________________

   5)  See also, Smith v. Magic City Kennel Club, Inc., 282 U. S. 784, 788
   5)            _____    _____________________________
(1931) (estoppel applied to amended claim where the original "claim was rejected
on the prior patent to" another); Computing Scale Co. of America v. Automatic
                                  ______________________________    _________
Scale Co., 204 U. S. 609, 618-620 (1907) (initial claims rejected based on lack
__________
of invention over prior patents); Hubbell v. United States, 179 U. S. 77, 83
                                  _______    ______________
(1900) (patentee estopped from excluding a claim element where element was added
to overcome objections based on lack of novelty over prior patents); Sutter v.
                                                                     ______
Robinson, 119 U. S. 530, 541 (1886) (estoppel applied where, during patent
________
prosecution, the applicant "was expressly required to state that [the device's]
structural plan was old and not of his invention"); cf. Graham v. John Deere Co.
                                                        ______    ______________
of Kansas City, 383 U. S. 1, 33 (1966) (noting, in a validity determination,
_______________
that "claims that have been narrowed in order to obtain the issuance of a patent
by distinguishing the prior art cannot be sustained to cover that which was
previously by limitation eliminated from the patent").

                                95-728 - OPINION

                    WARNER-JENKINSON v. HILTON DAVIS CHEMICAL                 13
                                    ____

tion that language it required would in many cases allow for a range of
equivalents, we should be extremely reluctant to upset the basic assumptions of
the PTO without substantial reason for doing so.  Our prior cases have consis-
tently applied prosecution history estoppel only where claims have been amended
for a limited set of reasons, and we see no substantial cause for requiring a
more rigid rule invoking an estoppel regardless of the reasons for a
change. (Ftnote. 6)
        (Ftnote. 6)
  In this case, the patent examiner objected to the patent claim due to a
perceived overlap with the Booth patent, which revealed an ultrafiltration
process operating at a pH above 9.0.  In response to this objection, the phrase
"at a pH from approximately 6.0 to 9.0" was added to the claim.  While it is
undisputed that the upper limit of 9.0 was added in order to distinguish the
Booth patent, the reason for adding the lower limit of 6.0 is unclear.  The
lower limit certainly did not serve to distinguish the Booth patent, which said
nothing about pH levels below 6.0.  Thus, while a lower limit of 6.0, by its
mere inclusion, became a material element of the claim, that did not necessarily
                                  _______
preclude the application of the doctrine of equivalents as to that element.  See
Hubbell v. United States, 179 U. S. 77, 82 (1900) ("`[A]ll [specified elements]
_______    ______________
must be regarded as mater-ial,'" though it remains an open " `question whether
an omitted part is supplied by an equivalent device or instrumentality'"
(citation omitted)).  Where the reason

























                    
____________________

   6)  That petitioner's rule might provide a brighter line for determining
   6)  
whether a patentee is estopped under certain circumstances is not a sufficient
reason for adopting such a rule.  This is especially true where, as here, the
PTO may have relied upon a flexible rule of estoppel when deciding whether to
ask for a change in the first place.  To change so substantially the rules of
the game now could very well subvert the various balances the PTO sought to
strike when issuing the numerous patents which have not yet expired and which
would be affected by our decision.

                                95-728 - OPINION

14                  WARNER-JENKINSON v. HILTON DAVIS CHEMICAL
                                    ____

for the change was not related to avoiding the prior art, the change may
introduce a new element, but it does not necessarily preclude infringement by
equivalents of that element. (Ftnote. 7)
                             (Ftnote. 7)
  We are left with the problem, however, of what to do in a case like the one at
bar, where the record seems not to reveal the reason for including the lower pH
limit of 6.0.  In our view, holding that certain reasons for a claim amendment
may avoid the application of prosecution history estoppel is not tantamount to
holding that the absence of a reason for an amendment may similarly avoid such
                 _______
an estoppel.  Mindful that claims do indeed serve both a definitional and a
notice function, we think the better rule is to place the burden on the patent-
holder to establish the reason for an amendment required during patent
prosecution.  The court then would decide whether that reason is sufficient to
overcome prosecution history estoppel as a bar to application of the doctrine of
equivalents to the element added by that amendment.  Where no explanation is
established, however, the court should presume that the PTO had a substantial
reason related to patentability for including the limiting element added by
amendment.  In those circumstances, prosecution history estoppel would bar the
application of the doctrine equivalents as to that element.  The presumption we
have described, one subject to rebuttal if an appropriate reason for a



























                    
____________________

   7)  We do not suggest that, where a change is made to overcome an objection
   7)  
based on the prior art, a court is free to review the correctness of that
objection when deciding whether to apply prosecution history estoppel.  As
petitioner rightly notes, such concerns are properly addressed on direct appeal
from the denial of a patent, and will not be revisited in an infringement
action.  Smith v. Magic City Kennel Club, Inc., supra, at 789-790.  What is per-
         _____    ____________________________  _____                    __
missible for a court to explore is the reason (right or wrong) for the objection
and the manner in which the amendment addressed and avoided the objection.

                                95-728 - OPINION

                    WARNER-JENKINSON v. HILTON DAVIS CHEMICAL                 15
                                    ____

required amendment is established, gives proper deference to the role of claims
in defining an invention and providing public notice, and to the primacy of the
PTO in ensuring that the claims allowed cover only subject matter that is
properly patentable in a proffered patent application.  Applied in this fashion,
prosecution history estoppel places reasonable limits on the doctrine of
equivalents, and further insulates the doctrine from any feared conflict with
the Patent Act.
  Because respondent has not proffered in this Court a reason for the addition
of a lower pH limit, it is impossible to tell whether the reason for that
addition could properly avoid an estoppel.  Whether a reason in fact exists, but
simply was not adequately developed, we cannot say.  On remand, the Federal
Circuit can consider whether reasons for that portion of the amendment were
offered or not and whether further opportunity to establish such reasons would
be proper.
                                        B
  Petitioner next argues that even if Graver Tank remains good law, the case
                                      ___________
held only that the absence of substantial differences was a necessary element
                                                            _________
for infringement under the doctrine of equivalents, not that it was sufficient
                                                                    __________
for such a result.  Brief for Petitioner 32.  Relying on Graver Tank's
                                                         ___________
references to the problem of an "unscrupulous copyist" and "piracy," 339 U. S.,
at 607, petitioner would require judicial exploration of the equities of a case
before allowing application of the doctrine of equivalents.  To be sure, Graver
                                                                         ______
Tank refers to the prevention of copying and piracy when describing the benefits
____
of the doctrine of equivalents.  That the doctrine produces such benefits,
however, does not mean that its application is limited only to cases where those
particular benefits are obtained.
  Elsewhere in Graver Tank the doctrine is described in more neutral terms.  And
               ___________
the history of the doctrine as

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16                  WARNER-JENKINSON v. HILTON DAVIS CHEMICAL
                                    ____

relied upon by Graver Tank reflects a basis for the doctrine not so limited as
               ___________
petitioner would have it.  In Winans v. Denmead, 15 How. 330, 343 (1854), we
                              ______    _______
described the doctrine of equivalents as growing out of a legally implied term
in each patent claim that "the claim extends to the thing patented, however its
form or proportions may be varied."  Under that view, application of the
doctrine of equivalents involves determin- ing whether a particular accused
product or process infringes upon the patent claim, where the claim takes
the form - half express, half implied - of "X and its equivalents."
  Machine Co. v. Murphy, 97 U. S. 120, 125 (1878), on which Graver Tank also
  ___________    _______                                    ___________
relied, offers a similarly intent-neutral view of the doctrine of equivalents:

   "[T]he substantial equivalent of a thing, in the sense of the patent law, is
   the same as the thing itself; so that if two devices do the same work in
   substantially the same way, and accomplish substantially the same result,
   they are the same, even though they differ in name, form, or shape."

If the essential predicate of the doctrine of equivalents is the notion of
identity between a patented invention and its equivalent, there is no basis for
treating an infringing equivalent any differently than a device that infringes
the express terms of the patent.  Application of the doctrine of equivalents,
therefore, is akin to determining literal infringement, and neither requires
proof of intent.
  Petitioner also points to Graver Tank's seeming reliance on the absence of
                            ___________
independent experimentation by the alleged infringer as supporting an equitable
defense to the doctrine of equivalents.  The Federal Circuit explained this
factor by suggesting that an alleged infringer's behavior, be it copying,
designing around a patent, or independent experimentation,

                                95-728 - OPINION

                    WARNER-JENKINSON v. HILTON DAVIS CHEMICAL                 17
                                    ____

indirectly reflects the substantiality of the differences between the patented
invention and the accused device or process.  According to the Federal Circuit,
a person aiming to copy or aiming to avoid a patent is imagined to be at least
marginally skilled at copying or avoidance, and thus intentional copying raises
an inference - rebuttable by proof of independent development - of having only
insubstantial differences, and intentionally designing around a patent claim
raises an inference of substantial differences.  This explanation leaves much to
be desired.  At a minimum, one wonders how ever to distinguish between the
intentional copyist making minor changes to lower the risk of legal action, and
the incremental innovator designing around the claims, yet seeking to capture as
much as is permissible of the patented advance.
  But another explanation is available that does not require a divergence from
generally objective principles of patent infringement.  In both instances in
Graver Tank where we referred to independent research or experiments, we were
___________
discussing the known interchangeability between the chemical compound claimed in
the patent and the compound substituted by the alleged infringer.  The need for
independent experimentation thus could reflect knowledge - or lack thereof - of
interchangeability possessed by one presumably skilled in the art.  The known
interchangeability of substitutes for an element of a patent is one of the
express objective factors noted by Graver Tank as bearing upon whether the
                                   ___________
accused device is substantially the same as the patented invention.  Independent
experimentation by the alleged infringer would not always reflect upon the
objective question whether a person skilled in the art would have known of the
interchangeability between two elements, but in many cases it would likely be
probative of such knowledge.
  Although Graver Tank certainly leaves room for
           ___________

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18                  WARNER-JENKINSON v. HILTON DAVIS CHEMICAL
                                    ____

petitioner's suggested inclusion of intent-based elements in the doctrine of
equivalents, we do not read it as requiring them.  The better view, and the one
consistent with Graver Tank's predecessors and the objective approach to
                ___________
infringement, is that intent plays no role in the application of the doctrine of
equivalents.
                                        C
  Finally, petitioner proposes 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.  A milder version of
this argument, which found favor with the dissenters below, is that the doctrine
should be limited to equivalents that were known at the time the patent was
issued, and should not extend to after-arising equivalents.
  As we have noted, supra, at 17, with regard to the objective nature of the
                    _____
doctrine, a skilled practitioner's knowledge of the interchangeability between
claimed and accused elements is not relevant for its own sake, but rather for
what it tells the fact-finder about the similarities or differences between
those elements.  Much as the perspective of the hypothetical "reasonable person"
gives content to concepts such as "negligent" behavior, the perspective of a
skilled practitioner provides content to, and limits on, the concept of
"equivalence."  Insofar as the question under the doctrine of equivalents is
whether an accused element is equivalent to a claimed element, the proper time
for evaluating equivalency - and thus knowledge of interchangeability between
elements -  is at the time of infringement, not at the time the patent was
issued.  And rejecting the milder version of petitioner's argument necessarily
rejects the more severe proposition that equivalents must not only be known, but
must also be actually disclosed in the patent in order for such equivalents to
infringe upon the patent.

                                95-728 - OPINION

                    WARNER-JENKINSON v. HILTON DAVIS CHEMICAL                 19
                                    ____

                                       IV
  The various opinions below, respondents, and amici devote considerable
                                               _____
attention to whether application of the doctrine of equivalents is a task for
the judge or for the jury.  However, despite petitioner's argument below that
the doctrine should be applied by the judge, in this Court petitioner makes only
passing reference to this issue.  See Brief for Petitioner 22, n. 15 ("If this
Court were to hold in Markman v. Westview Instruments, Inc., No. 95-26 (argued
                      _______    ___________________________
Jan. 8, 1996), that judges rather than juries are to construe patent claims, so
as to provide a uniform definition of the scope of the legally protected
monopoly, it would seem at cross-purposes to say that juries may nonetheless
expand the claims by resort to a broad notion of `equivalents'"); Reply Brief
for Petitioner 20 (whether judge or jury should apply the doctrine of
equivalents depends on how the Court views the nature of the inquiry under the
doctrine of equivalents).
  Petitioner's comments go more to the alleged inconsistency between the
doctrine of equivalents and the claiming requirement than to the role of the
jury in applying the doctrine as properly understood.  Because resolution of
whether, or how much of, the application of the doctrine of equivalents can be
resolved by the court is not necessary for us to answer the question presented,
we decline to take it up.  The Federal Circuit held that it was for the jury to
decide whether the accused process was equivalent to the claimed process.  There
was ample support in our prior cases for that holding.  See, e.g., Machine Co.
                                                             ____  ___________
v. Murphy, 97 U. S., at 125 ("in determining the question of infringement, the
   _______
court or jury, as the case may be, . . . are to look at the machines or their
several devices or elements in the light of what they do, or what office or
function they perform, and how they perform it, and to find that one thing is
substantially the same as another, if it performs

                                95-728 - OPINION

20                  WARNER-JENKINSON v. HILTON DAVIS CHEMICAL
                                    ____

substantially the same function in substantially the same way to obtain the same
result"); Winans v. Denmead, 15 How., at 344 ("[It] is a question for the jury"
          ______    ________
whether the accused device was "the same in kind, and effected by the employment
of [the patentee's] mode of operation in substance").  Nothing in our recent
Markman decision necessitates a different result than that reached by the
_______
Federal Circuit.  Indeed, Markman cites with considerable favor, when discussing
                          _______
the role of judge and jury, the seminal Winans decision.  Markman v. Westview
                                        ______            _______    ________
Instruments, Inc., 517 U. S. ___ (1996) (slip op., at 14).  Whether, if the
__________________
issue were squarely presented to us, we would reach a different conclusion than
did the Federal Circuit is not a question we need decide
today. (Ftnote. 8)
       (Ftnote. 8)





















                    
____________________

   8)  With regard to the concern over unreviewability due to black-box jury
   8)  
verdicts, we offer only guidance, not a specific mandate.  Where the evidence is
such that no reasonable jury could determine two elements to be equivalent,
district courts are obliged to grant partial or complete summary judgment.  See
Fed. Rule Civ. Proc. 56; Celotex Corp. v. Catrett, 477 U. S. 317, 322-323
                         _____________    ________
(1986).  If there has been a reluctance to do so by some courts due to un-
familiarity with the subject matter, we are confident that the Federal Circuit
can remedy the problem.  Of course, the various legal limitations on the
application of the doctrine of equivalents are to be determined by the court,
either on a pretrial motion for partial summary judgment or on a motion for
judgment as a matter of law at the close of the evidence and after the jury
verdict.  Fed. Rule Civ. Proc. 56; Fed. Rule Civ. Proc. 50.  Thus, under the
particular facts of a case, if prosecution history estoppel would apply or if a
theory of equivalence would entirely vitiate a particular claim element, partial
or complete judgment should be rendered by the court, as there would be no
further material issue for the jury to resolve.  Finally, in cases that reach
        ________
the jury, a special verdict and/or interrogatories on each claim element could
be very useful in facilitating review, uniformity, and possibly postverdict
judgments as a matter of law.  See Fed. Rule Civ. Proc. 49; Fed. Rule Civ. Proc.
50.  We leave it to the Federal Circuit how best to implement procedural im-
provements to promote certainty, consistency, and reviewability to this area of
the law.

                                95-728 - OPINION

                    WARNER-JENKINSON v. HILTON DAVIS CHEMICAL                 21
                                    ____

                                        V
  All that remains is to address the debate regarding the linguistic framework
under which "equivalence" is determined.  Both the parties and the Federal
Circuit spend considerable time arguing whether the so-called "triple identity"
test - focusing on the function served by a particular claim element, the way
                       ________                                           ___
that element serves that function, and the result thus obtained by that element
                                           ______
- is a suitable method for determining equivalence, or whether an "insubstantial
differences" approach is better.  There seems to be substantial agreement that,
while the triple identity test may be suitable for analyzing mechanical devices,
it often provides a poor framework for analyzing other products or processes. 
On the other hand, the insubstantial differences test offers little additional
guidance as to what might render any given difference "insubstantial."
  In our view, the particular linguistic framework used 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.  A focus on individual
elements and a special vigilance against allowing the concept of equivalence to
eliminate completely any such elements should reduce considerably the impreci-
sion of whatever language is used.  An analysis of the role played by each
element in the context of the specific patent claim will thus inform the inquiry
as to whether a substitute element matches the function, way, and result of the
claimed element, or whether the substitute element plays a role substantially
different from the claimed element.  With these limiting princi-

                                95-728 - OPINION

22                  WARNER-JENKINSON v. HILTON DAVIS CHEMICAL
                                    ____

ples as a backdrop, we see no purpose in going further and micro-managing the
Federal Circuit's particular word-choice for analyzing equivalence.  We expect
that the Federal Circuit will refine the formulation of the test for equivalence
in the orderly course of case-by-case determinations, and we leave such refine-
ment to that court's sound judgment in this area of its special expertise.
                                       VI
  Today we adhere to the doctrine of equivalents.  The determination of
equivalence should be applied as an objective inquiry on an element-by-element
basis.  Prosecution history estoppel continues to be available as a defense to
infringement, but if the patent-holder demonstrates that an amendment required
during prosecution 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, a court should
presume that the purpose behind the required amendment is such that prosecution
history estoppel would apply.  Because the Court of Appeals for the Federal
Circuit did not consider all of the requirements as described by us today,
particularly as related to prosecution history estoppel and the preservation of
some meaning for each element in a claim, we reverse and remand for further
proceedings consistent with this opinion.
                                                               It is so ordered.
                                                               ________________