delivered the opinion of the Court.
The question for our decision is whether a patent licensee, by virtue of his license agreement, is estopped to challenge a price-fixing clause in the agreement by showing that the patent is invalid, and that the price restriction is accordingly unlawful because not protected by the patent monopoly.
Respondent brought the present suit in the District Court for the Northern District of Illinois, asserting di
Petitioner by its answer admitted that it had manufactured two types of transformers, one covered by certain narrow claims of the patent, claims 8,14 and 19, the validity of which it does not challenge, the other alleged to be covered by certain broader claims. Petitioner also filed a counterclaim alleging that the broad claims are invalid for want of novelty, as it asserted had been recognized in the Sixth Circuit in
France Mfg. Co.
v.
Jefferson Electric Co.,
The Circuit Court of Appeals for the Seventh Circuit affirmed the District Court’s order dismissing the counterclaim,
The Circuit Court of Appeals, in holding that petitioner as a licensee was estopped to challenge the validity of the patent, did not say whether it considered that it was applying a rule of federal or of state law, and it cited no decisions of either the federal or the Illinois courts. Where no price-fixing stipulation was involved in the license contract, this rule of estoppel, which was not questioned by counsel, was applied without discussion in
United States
v.
Harvey Steel Co.,
The present license contract contemplates and requires that petitioner, on sales of the licensed transformers throughout the United States, shall conform to the prices fixed by respondent for the sale of competing patented articles by other licensees and by respondent. Such a restriction on the price of articles entering interstate commerce is a violation of the Sherman Act save only as it is within the protection of a lawfully granted patent monopoly. See
United States
v.
Univis Lens Co.,
It is familiar doctrine that the prohibition of a federal statute may not be set at naught, or its benefits denied, by state statutes or state common law rules. In such a case our decision is not controlled by
Erie R. Co.
v.
Tompkins,
The federal courts have been consistent in holding that local rules of estoppel will not be permitted to thwart the purposes of statutes of the United States. See, in the case of federal statutes governing interstate freight rates,
Pittsburgh, C., C. & St. L. Ry. Co.
v.
Fink,
A state by applying its own law of specific performance may not compel the performance of a contract contemplating violation of the federal land laws,
Anderson
v.
Carkins,
Local rules of estoppel which would fasten upon the public as well as the petitioner the burden of an agreement in violation of the Sherman Act must yield to the Act's declaration that such agreements are unlawful, and to the public policy of the Act which in the public interest precludes the enforcement of such unlawful agreements. Cf.
Morton Salt Co.
v.
Suppiger Co.,
Reversed.
