ASHINGTON, June 14 - The Supreme Court ruled Monday in a closely watched international price-fixing case that federal antitrust law does not apply to transactions that take place overseas and that cause harm there unless a company's actions in the United States can be shown to have contributed to the harm.

The decision, 8 to 0, overturned a ruling last year by the United States Court of Appeals for the District of Columbia Circuit that had caused considerable alarm among multinational corporations and foreign governments by significantly expanding the reach of American antitrust law and the jurisdiction of the federal courts to hear such cases.

The Bush administration had urged the justices to overturn the appeals court's ruling, which came in a suit by foreign customers of vitamin manufacturers and distributors that were the subjects of a United States investigation in the late 1990's into international price-fixing. That investigation led to guilty pleas and to the biggest criminal fine, $500 million, ever obtained by the Justice Department, in addition to heavy civil penalties imposed by the European Union and several foreign governments.

The case before the Supreme Court on Monday was a private lawsuit seeking triple damages against the same defendants. It was brought by five companies in Australia, Ecuador, Panama and Ukraine as a class action on behalf of overseas purchasers who use vitamins in a range of commercial applications, like animal feed and additions to breakfast cereal. Stalled by the issue of jurisdiction, the private lawsuit has not yet gone to trial.

Writing for the court, Justice Stephen G. Breyer said that the Sherman Antitrust Act does not cover the foreign effects of anticompetitive conduct unless the defendants' domestic behavior can be shown to have contributed to those effects.

In this case, F. Hoffmann-LaRoche Ltd. v. Empagran S.A., No. 03-724, the five foreign plaintiffs contended that the companies' ability to raise prices in the United States did in fact contribute to antitrust injury abroad. Their lawyer, Thomas C. Goldstein, said in an interview Monday that his clients consequently could meet the Supreme Court's new test as the case goes forward.

Because the court of appeals did not address the question of domestic effects, Justice Breyer said the Supreme Court could not consider it at this stage. The case now returns to the appeals court.

Stephen A. Bokat, senior vice president and general counsel of the United States Chamber of Commerce, said the decision was a "very positive development" in its campaign against what it calls "global forum shopping."

The decision interpreted a 1982 law, an amendment to the Sherman Act called the Foreign Trade Antitrust Improvement Act. While it generally provides that federal antitrust law will not apply to foreign trade or commerce, it contains an exception for conduct that has a "direct, substantial, and reasonably foreseeable effect" on domestic commerce.

"Where the plaintiff's claim rests solely on the independent foreign harm," that exception does not apply, Justice Breyer said. Justice Sandra Day O'Connor did not participate in the case.

Canada, Germany and Japan filed briefs warning the court that application of United States antitrust law would interfere with foreign governments' efforts to regulate their own markets. The Bush administration's brief on the same side was signed both by the usual contingent of Justice Department lawyers and by the State Department's principal legal adviser, William H. Taft IV.