The United Auto Workers union said yesterday that it will concentrate its negotiations on General Motors Corp. as it attempts to reach a new national labor contract covering 443,000 active workers at the nation's three largest car companies.

Getting selected as the so-called "strike target" is considered a coup, since it gives GM the ability to customize the terms of an agreement that the UAW will then take to Ford Motor Co. and Chrysler Corp., the other two companies involved in the talks.

It also means that GM will be the company that the auto workers strike if an agreement is not reached by midnight Sept. 14, when the current contracts expire, although neither side is anticipating a strike this year.

The news appeared to help boost GM's shares on Wall Street yesterday. They climbed $1.12 1/2 to close at $40.12 1/2 and GM was the most active stock on the New York Stock Exchange.

The UAW targeted GM largely because the union has the most jobs at risk there. The company has 280,000 active UAW-represented workers, compared with 99,800 at Ford and 63,285 at Chrysler.

Union strategists figure that Ford and Chrysler have already made substantial reductions in their work force -- primarily by eliminating jobs in their auto parts operations. Consequently, it would be more difficult to regain those jobs at Ford and Chrysler and easier to try to slow GM's move in that direction, analysts say.

According to the auto consulting firm Harbour & Associates Inc. of Troy, Mich., 70 percent of all parts going into GM's products in North America are made by UAW workers in GM plants, compared with 50 percent UAW-made parts at Ford and 38 percent UAW-made parts at Chrysler.

But GM's huge amount of in-house parts production has also helped to saddle the company with the biggest production costs in North America. That means GM is under tremendous pressure from Wall Street to cut its components work force, or to work out some other agreement with the UAW that will allow the company to improve its competitive posture without scrapping jobs.

A strong agreement at GM could be used to halt the erosion of components jobs -- at least at Ford, analysts said.

GM is also being targeted because the UAW believes GM has the most to lose in a strike and will consequently negotiate to avoid one. In the past few months, GM has made some modest gains in market share after losing share for nearly a decade. Analysts say that GM neither wants to lose that momentum nor does it want to disrupt the launch of its highly publicized Saturn cars, due to go on sale in the fall.

Chrysler, which has suffered drastic declines in sales, market share and profits over the last two years, may be too weak to fit into any pattern that is acceptable to GM and Ford. For that reason, the UAW never considered Chrysler as a strike target in the current talks, said Bob Bernstein, an analyst with Edward D. Jones and Co. in Missouri.

"It was either Ford or GM, not Chrysler," Bernstein said. The union shied away from targeting Chrysler "because it did not want to drive it over the brink," Bernstein said.

The UAW had flirted with the idea of targeting Ford, partly because Ford's U.S. plants are operating at near full capacity and partly because Ford, in recent talks, has demonstrated a desire to settle contracts quickly.

But a settlement at Ford would do nothing to regain lost auto parts jobs.

Numbers form the basis of the union's concern. Between 1979 and 1988, the UAW employment at Big Three vehicle assembly plants in the United States fell 14.5 percent to 250,100 workers from 292,000, according to the latest available figures from Harbour & Associates. By comparison, UAW employment at Big Three parts plants fell 31.3 percent to 298,000 from 434,000, mostly because Chrysler and Ford began farming out parts work.

UAW employment at independent parts companies was hardest hit, falling 44 percent to 85,100 workers from 152,100 between 1979 and 1988, according to the Harbour report.

UAW President Owen Bieber and GM officials expressed belief that an agreement could be reached without a strike, a notion shared by other industry observers.

"There's not going to be a strike. Nobody can afford a strike. I think both sides will be willing to talk and that they will arrive at an agreement," said Patrick E. Sheridan, an analyst at McDonald & Co. in Cleveland.

Auto industry consultants and others familiar with the talks said that GM and the union already have completed most of the work on a new agreement, and they are now focusing on one big issue -- job security.