The recent strikes at Northwest Airlines, General Motors, Bell Atlantic and US WEST represent the biggest surge in major walkouts in years and, industrial relations experts say, point to a new aggressiveness on the part of the nation's labor unions.

For years unions were so discouraged by several disastrous strikes, most notably the Caterpillar and the air controllers' strikes, that they had generally shunned large-scale walkouts. And even though union membership rolls are slipping and the slowing economy threatens workers, unions have grown bolder, labor experts say, because of the tight labor market, more combative union leadership and the Teamsters' gains in last year's UPS strike.

In 1996, the number of strikes nationwide had fallen to its lowest level in 50 years, 374, before rising to 394 last year. Even before the prominent strikes of this summer, the number of work stoppages increased this year, rising to 226 in the first six months from 214 the first six months of last year, according to the Bureau of National Affairs.

Even more significant for labor, striking unions have emerged more successful than in recent years, pressuring US WEST, for instance, to scale back mandatory overtime and Bell Atlantic to stop making most new jobs nonunion.

"All this represents a more confident and aggressive labor movement," said Harley Shaiken, a professor of industrial relations at the University of California at Berkeley. "In the '80s, so many visible, pivotal strikes ended somewhere between disaster and debacle, but what we've seen this year is a number of important successes for unions."

But many business leaders assert that the recent eruption of strikes stems not from an emboldened labor movement but from coincidence - many collective bargaining agreements in industries with volatile labor relations have expired this summer.

"I don't see a trend," said Patrick Cleary, vice president for human resources policy at the National Association of Manufacturers. "I don't get a sense of any change in the national Zeitgeist."

But one cause for the surge in strikes is that many corporations continue to demand concessions. For instance, the 34,000 workers at US WEST went on strike after the company insisted on replacing the across-the-board pay increases with a pay-for-performance system, which would give management total discretion over each worker's raises - an idea abhorred by unions. And the GM strike began after the company refused to make good on promises to invest $300 million in a metal-stamping plant in Flint, Mich.

"All of these strikes show unions' willingness to fight back more," said Richard Hurd, a professor of labor relations at Cornell University. "But it's not clear to me whether these strikes are caused by newfound aggressiveness on the part of unions or newfound aggressiveness on the part of employers."

John Sweeney, president of the AFL-CIO, attributes the increase in strikes to both sides: greater labor confidence and to management hard-head-ed-ness. He said it was understandable that workers opt to strike when many corporations are making record profits and union members feel they are not getting their fair share.

Sweeney called the Northwest Airlines pilots' strike an example of this, asserting that the airline had made a stingy offer to its 6,200 pilots despite the company's booming profits and soaring executive pay. But the airline maintains that it cannot meet the pilots' demands, which management says would make them the nation's highest-paid pilots.

"What we're seeing is a complete lack of respect for workers, for the jobs they do," Sweeney said. "We're seeing how insensitive corporate America can be."