An American threat to retaliate against Japan for its aggressive system of exporting to the United States would be a mistake, U.S. Ambassador Mike Mansfield warned today.

The former Senate majority leader said that growing protectionist feeling in Congress may produce a "retaliation" against Japanese products that would be dangerous for both countries.

Mansfield's warning came at a meeting with American correspondents that reflected Japanese concern that the United States may put heavy pressure on Japan to reduce exports to the United States and increase imports of American products to reduce the current trade imbalance.

Richard Cooper, under secretary of state for economic affairs, and Fred Bergsten, assistant secretary of the treasury, will be in Tokyo next week for economic talks that are to focus on ways of bringing U.S. Japanese trade into better balance.

Their mission has been described in the Japanese press as a device to pressure the government into making trade concessions.

Mansfield said he does not expect the discussions to result in pressures on Japan.

"If it's going to be a case of retaliation, I think that would be a mistake," he said.

He said he hopes the talks will lead to an "early-warning system" that could alert both countries to economic problems before they get out of control and became politically dangerous.

The Japanese government has contended in a number of recent statements that Japan has not erected unfair barriers against American imports. The Japanese have suggested that the main problem is the lack of initiative on the part of American businessmen in opening up markets.

Mansfield expressed some sympathy for that view.

"I think Japan will allow more imports if American businessmen try harder," he said.

He also credited Japan with making some effort to restrict exports to the United States. Automobile manufacturers, he said, are trying to curb the sales of some models that have captured big segments of the American small-car market. One of the problems, he said, is that American dealers keep asking for more cars.

Mansfield said he sees little chance that American carmakers can sell large numbers of automobiles in Japan in the near future. Japan's tougher air pollution standards and its production of high-mileage cars make it hard for American manufacturers to compete, he said.

"The Japanese are very good businessmen," Mansfield said. "They make good products at competitive prices."