## Facts

- Hilton Davis owned U.S. Patent No. 4,560,746 covering a dye purification process using ultrafiltration through a membrane at pH levels between 6.0 and 9.0.
- During patent prosecution, the applicant added the 6.0–9.0 pH limitation after an examiner raised concerns about overlap with prior art operating at pH above 9.0.
- Warner-Jenkinson developed an ultrafiltration process operating at pH 5.0.
- Hilton Davis conceded there was no literal infringement because pH 5.0 fell outside the claimed range and pursued infringement solely under the doctrine of equivalents.
- The district court submitted equivalence to the jury, which found infringement under the doctrine of equivalents, and the court entered a permanent injunction.
- The Federal Circuit (en banc) affirmed, holding the doctrine of equivalents remained available and that substantial evidence supported the jury’s finding.

## Issues

1. Whether the doctrine of equivalents survived the 1952 Patent Act.
2. Whether equivalence must be determined for the invention as a whole or on an element-by-element basis.
3. How prosecution history estoppel limits the doctrine of equivalents when claim scope is narrowed during prosecution.
4. Who bears the burden to explain the reason for a narrowing amendment when the prosecution record is unclear, and what presumption applies if the reason is not established.

## Decision

- The Supreme Court held the doctrine of equivalents remains valid and was not eliminated by the 1952 Patent Act.
- The Court required equivalence to be evaluated on an element-by-element basis, treating each claim limitation as material.
- The Court held that prosecution history estoppel can bar reliance on equivalents for subject matter surrendered by narrowing amendments related to patentability.
- When the reason for a narrowing amendment is not clear from the record, a rebuttable presumption arises that the patentee surrendered the territory between the original and amended claim scope; the patentee bears the burden to rebut by showing the amendment was not made for reasons related to patentability.
- The Court reversed and remanded for reconsideration under the clarified standards.

## Legal Principles

- The doctrine of equivalents permits a finding of infringement even without literal claim coverage, but it may not be used to effectively eliminate a claim limitation.
- Equivalence must be assessed limitation-by-limitation, not by comparing the accused process to the invention in the aggregate.
- Prosecution history estoppel limits equivalents where a patentee narrows a claim to obtain allowance or avoid prior art; the patentee is generally barred from recapturing surrendered subject matter through equivalents.
- If the prosecution history does not reveal why a narrowing amendment was made, courts presume surrender of the intermediate scope, subject to the patentee’s rebuttal showing a non-patentability reason for the amendment.
- Factual determinations relevant to equivalence may be resolved by a jury, subject to correct legal instructions on element-by-element analysis and estoppel.

## Conclusion

The Court reaffirmed the doctrine of equivalents but tightened its application by requiring element-by-element analysis and strengthening prosecution history estoppel through a presumption that unexplained narrowing amendments surrender the intervening subject matter, leading to reversal and remand for application of these rules.


Warner-Jenkinson Co. v. Hilton Davis Chem. Co., 520 U.S. 17 (1997)