Citing new evidence of growing protectionism in the U.S. Congress, American officials warned Japan yesterday it must open its markets for imports or face new restrictions on its own products.

In a speech here, Commerce Secretary Juanita M. Kreps commended the Japanese for removing some trade barriers but said "the real test will be Japan's readiness to open her markets on an industry-wide basis. The alternative of protectionism is in nobody's interest - least of all Japan's."

Kreps said Japan historically has refused to take responsibility for maintaining an open trading system from which it has benefited conspicuously in recent years.

The cabinet officer and other officials arrived here Monday night accompanying a large trade mission of American businessmen hoping to discover new customers in Japan.

In a series of private meetins, news conferences and speeches they stressed that protectionism sentiment is rising and that the best way to deflate it is for Japan to reduce its large trade surplus by buying American.

The discussions around Tokyo yesterday showed that American officials are using the big trade mission as a kind of litmus test of Japanese promises to lower trade barriers. Japan claims that American businessmen don't try hard enough to sell in this country. The U.S. mission is in part a test of that argument.

Assistant Secretary of Commerce Frank Weil said he told the Japanese Minister of Trade and Industry, Toshio Komoto, that Americans hope to see evidence that his government will change its policies on three fronts - trading cartels, the government's "administrative guidance" on trade and Japanese government procurement.

"I told Komoto that we need the cooperation of the Japanese government as evidence that the Japanese market is accessible to American businessmen," Weil said. He said Komoto seemed eager to provide some evidence.

Japan has had a trade surplus with the U.S. for the past 13 years and last year racked up a world-wide surplus in current accounts amounting to about $14 billion. The government had promised to cut that back to about $6 billion but already acknowledges that it will be nearly as high as last year, even though its volume of exports has begun to decline.

The U.S. has repeatedly charged that Japan excludes exports artificially through a system of tariffs, quotas, rules and regulations that make selling in this country difficult. By perpetuating large trade imbalances, Japan endangers the world trade system, U.S. officials claim.

Kreps, in a speech to foreign correspondents, said no country "is free to pursue policies that result in chronic imbalances vis-a-vis the community of trading nations as a whole.

"When one country imagines its self-interest to lie in winning the trade numbers game rather than in preserving the health of the system that makes that trade possible, both objectives are threatened," she said.

She took note of the Japanese contention that American businessmen are complacent about exports and agreed that this partly explains the continuing trade imbalances.

"Of greater importance has been an historical unwillingness of the Japanese government to shoulder its responsibilities under the open trading system of which Japan has been in recent years the most conspicuous beneficiary," she said.

There is evidence that that attitude has changed, she acknowledged, citing the government's stimulation of the domestic economy and prime minister Fukuda's commitment to keep the volume of Japanese exports below that of the last fiscal year.

Kreps said yesterday the mission has set no specific target for sales on this trip.