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

The Doctrine of Equivalents says equivalents of the claimed elements in a patent claim can also cause infringement.  Judge Learned Hand said that courts "resort to the 'doctrine of equivalents' to temper unsparing logic and prevent an infringer from stealing the benefit of the invention.”  Royal Typewriter co. v. Remington Rand, Inc., 168 F.2d 691, 692 (2d Cir. 1948) One of the limits on the Doctrine of Equivalents is prosecution history estoppel. Prosecution history estoppel is the presumption that any narrowing limitation added to the claims during prosecution bars the doctrine of equivalents in defining scope of the amended claims.

In Warner-Jenkinson a chemical process was patented.  During prosecution a limitation was added that a step in the chemical process had to be carried out at a PH of between six and nine.  The upper limit, nine, was added to overcome the prior art of record; but at the time of trial, it was unclear why the lower limit, of six, was added to the claimed chemical process.  The alleged infringer was doing the same chemical process at a PH of two and the patent owner sued the infringer arguing that under the doctrine of equivalents, the infringer was using an insubstantial tweak to the chemical process to avoid literal infringement.  The Supreme court decided that prosecution history should be used to limit the doctrine of equivalents whenever an amendment was made for a “substantial reason related to patentability…” and left it up to lower courts to decide what a substantial reason related to patentability is.  Waner-Jenkinson also did not decide whether prosecution history estoppel was a complete bar to Doctrine of Equivalents or whether prosecution history estoppel just limited the number of things that could be considered equivalents.