The United Auto Workers union last night accepted concessions that American Motors Corp. said were necessary to keep open AMC's lone auto plant in the United States.

AMC, America's fourth-largest auto maker in terms of 1984 production, has been under pressure from its chief investor to bring its costs in line with those of General Motors Corp., the nation's biggest car company.

AMC is 46.4 percent owned -- and controlled -- by auto maker Regie Nationale des Usines Renault of France.

"We believe that it is a competitive contract. It's an agreement that we can live with," said AMC manufacturing spokesman Lloyd A. Northard, commenting on the new UAW contract.

If no agreement had been reached by midnight yesterday, AMC would have proceeded with plans to close its Kenosha, Wisc., plant -- in operation since 1902 -- by July 1, 1986. Related operations at a facility in Milwaukee would have ceased in mid-September 1985.

Following the agreement, Ralph Koenig, director of UAW Region 10, said AMC "has not bargained in good faith" and that many elements of its offer were "abhorrent. However, we don't see how urging the membership to reject the agreement would improve it in any way," The Associated Press reported.

Koenig said the executive boards of Local 72, at Kenosha, and Local 75, at Milwaukee, voted unanimously to urge acceptance.

Rudy Kuzel, president of Local 72, and Nick Romano, president of Local 75, predicted that the members would ratify the contract to save their jobs.

A ratification vote is scheduled to take place the week of July 8.

UAW Secretary-Treasurer Ray Majerus said the contract's economic provisions matched others in the industry but that its proposals for working conditions were disappointing.

Richard Calmes, AMC vice president for personnel and human relations, said the final proposal included "significant changes" in response to concerns expressed at the bargaining table.

Renault pumped $545 million into AMC's faltering operations between October 1979 and March 1985. The French company, over the objections of its home-based unions, said yesterday it would lend another $175 million to AMC to keep the company rolling.

But Renault officials say they cannot keep operating AMC's sole U.S. auto plant at Kenosha, which produces the Renault Alliance and Encore models, and the Milwaukee facility as long as those plants have labor costs exceeding those of GM. The two plants employ 7,400 workers.

AMC's Kenosha workers, for example, receive an estimated 38 cents more an hour than their counterparts at comparable GM plants.

Slumping sales of subcompact cars -- the only kind AMC produces in this country -- are adding to the little auto maker's woes. AMC lost $29 million in the first three months of 1985, ending five quarters of modest profitability.

The company is expected to show a similar earnings decline in the second quarter of this year.

AMC in May warned the UAW that it would shut the Wisconsin operations if the union did not agree to a series of concessions designed to reduce costs.

It was the kind of threat AMC had avoided in 30 years of dealing with the union, largely because the company's one U.S. plant was its sole source of passenger cars. But the Renault link has given AMC the option of importing cars from France for U.S. sales..

Kenosha and UAW officials strongly objected to the manner in which the company stated its demands.