The U.S. Supreme Court made it more difficult yesterday for U.S. firms to use antitrust laws to fight foreign competition, ruling that American companies must have substantial evidence that foreign firms conspired to monopolize the market.

The 5-to-4 ruling in Matsushita Electric Industrial Co. Ltd. v. Zenith Radio Corp., a major victory for Japanese television makers in a 15-year-battle with their U.S. competitors, also may make it easier for antitrust defendants in general to have suits against them dismissed before trial, antitrust experts said yesterday.

The decision by Justice Lewis F. Powell Jr. reverses a ruling by an appeals court in Philadelphia that ordered 21 Japanese companies to stand trial on allegations that they conspired to sell television sets in the United States below actual cost in order to drive U.S. television manufacturers out of business.

The American firms, Zenith Electronics Corp. and National Union Electric Co., argued that the Japanese firms -- including Toshiba, Sanyo, Sharp and Mitsubishi, but not Sony -- had acted together to sell televisions in Japan at artificially high prices to offset low prices in America.

Powell said the evidence presented by the U.S. companies to support their allegations was insufficient to allow the case to go to trial. "A conspiracy to increase profits in one market does not tend to show a conspiracy to sustain losses in another," Powell said. Such "predatory pricing conspiracies," he said, "are generally unlikely to occur . . . especially where, as here, the prospects of attaining monopoly power seem slight."

Powell said that while the Japanese manufacturers' shares of the market rose from 20 percent to 50 percent during the 1960s and 1970s, "the alleged conspiracy's failure to achieve its ends after 20 years is strong evidence that the conspiracy does not in fact exist."

In addition, Powell said that the Japanese firms had no "motive to sustain such losses by underpricing in the United States absent some strong likelihood that the alleged conspiracy in this country will eventually pay off" in market control and an ability to later inflate prices.

The court sent the case back to the lower court to see if the American companies could produce more direct evidence of a pricing conspiracy. At the same time, the American companies could pursue their other claims that the Japanese manufacturers violated U.S. laws against "dumping" their goods at below-market prices.

A spokesman for Zenith said company attorneys "feel the antitrust case is still alive" and especially the antidumping case. The company was "looking forward to trying the dumping case in the near future," the spokesman said.

But Donald J. Zoeller, an attorney for the Japanese firms, said the ruling will put those companies in a "tremendous position to have all the conspiracy charges dismissed -- both on antitrust and antidumping."

Although Zenith may try to pursue allegations of dumping against individual Japanese companies, Zoeller said, the ruling yesterday should serve to defeat those efforts as well.

Justice Byron R. White, writing in dissent for Justices William J. Brennan Jr., Harry A. Blackmun and John Paul Stevens, said the American companies had presented enough evidence to send the case to trial. White said the court, trying to establish guidelines for judges to decide when to dismiss antitrust suits before trial, "only muddies the waters . . . with confusing and inconsistent statements about the appropriate standard for granting summary judgment."

The Japanese companies, backed by the Justice Department, also had argued that they were exempt from antitrust laws because whatever actions they took were required by the government of Japan. The court did not address that issue.