The newest members of the U.S. work force are called everything from "disposable workers" to "just-in-time employees" to "flexforce workers."

But no matter what label you use, they have become part of a sometimes destabilizing, contentious change permeating the world of work.

For years, U.S. corporations cut large numbers from their payrolls to reduce costs and improve efficiency. But even as the economy has strengthened, a growing number are turning to armies of temporaries, contractors and part-timers.

Some employees prefer the flexibility of so-called "contingent" work. However, many are being swept up by a "temping of America" that is turning long-standing assumptions about jobs upside down.

Employers seem to be moving toward a two-tiered work force in which a core of essential full-time employees is supplemented by contingent workers.

In the process, middle-class expectations of a job that carries pension, sick leave, vacation, health insurance and a prospect of upward mobility may be irreversibly slipping away, some analysts say.

Consider, for instance, that the largest private employer in the nation last year was Manpower Inc., the Milwaukee-based temporary help firm that dispatched 640,000 workers on short-term assignments. That surpassed even General Motors Corp., whose 330,000 full-time U.S. workers made it the largest manufacturing employer.

"This is the work force equivalent of a one-night stand. There's no long-term commitment" from either the employee or the employer, says Richard S. Belous, chief economist of the National Planning Association, a research group in Washington. However, he notes, the trend also produces a big plus, providing workers, employers and society at large with tremendous flexibility.

Many in industry praise the concept of a flexible work force, saying that, in an era of cutthroat global competition, U.S. companies need to be lean and must be able to adjust staffing to meet the ever-changing demands of their markets.

"If they couldn't be flexible, they couldn't compete, and they would go out of business," says Herb Cogliano, vice president of Sullivan & Cogliano Cos., a temporary help business in Waltham, Mass. "It's the realities of the new world economy."

Meanwhile, the trend forces legions of workers to carve out a risky, uncertain existence. Often without health insurance or pensions, they are modern-day nomads, roving from assignment assignment.

Some critics go further, saying that contingent work creates second-class workers, who earn less and lack the benefits, training and career advancement opportunities of permanent full-timers.

Says Karen Nussbaum, director of the women's division at the U.S. Labor Department: "We're really breaking those assumptions about the relationship between the worker and the enterprise. It puts an incredible strain on working people."

Ms. Nussbaum, who follows the issue closely because so many contingent workers are women, says the shift toward contingent work may be as profound as the change society underwent when workers went from farms to factories.

Worried that the trend will undermine its goal of "high wage, high skill" jobs in the rebounding economy, the Clinton administration is conducting an interagency study on contingent work. Specifically, the Labor Department is looking at whether federal regulations on pensions, unemployment insurance and other programs leave gaps in coverage for contingent workers.

Nowhere is the shift more visible than in the fast-growing $28 billion temporary help industry. The nation's temp agencies dispatched an average of 1.6 million workers a day last year, more than three times the 472,000 in 1983, according to the National Association of Temporary Services. Although temps -- defined as those hired out through temporary employment agencies -- represented only 1.4 percent of the work force last year, that was up from 0.5 percent in 1983.

When other workers, such as contractors and part-timers -- whose ranks are being increased by people who would prefer full-time work -- are included, the contingent work force swells to 34 million, or even higher, according to some estimates. That would mean 1 out of 4 U.S. workers do not have traditional full-time jobs.

Some people think such numbers are too high. They note the lack of consensus on what exactly constitutes a contingent worker. For example, some statistics indicate that as many as two-thirds of part-time workers are voluntary part-timers. Mothers and students have long been a significant part of the part-time work force. And some skeptics question whether self-employed workers should be counted among the contingent work force.

Nonetheless, most agree that contingent workers span the wage and skills spectrum. They include secretaries, clerks, janitors and, increasingly, professionals such as computer graphics specialists, lab technicians, doctors and even chief executives.

Certainly, there are plenty of people, such as working parents, who seek the freedom and flexibility of short-term jobs. Jan Cellana of Marion, Mass., has been an independent human resources consultant to large companies, such as Lotus, Reebok and Bank of Boston. She views contracting as a career -- not a stopgap -- because she wants time for her 2-year-old daughter.

"It becomes a quality-of-life issue. The company doesn't own me and doesn't own my time. I work a basic eight- or nine-hour day. I cannot and will not work 12 hours a day," says Ms. Cellana, a single mother. "I'm not interested in career advancement. When you contract, you think about your career in a different way."

Temping can also serve as good experience between jobs or during career changes.

Ken Cefalo, 36, of Winchester, Mass., sees his recent temp assignment as a lab technician at a Good Humor-Breyers ice cream plant as a way to help him secure a permanent position in a microbiology lab. "This is an excellent avenue to get experience to put on your resume," says Mr. Cefalo, who earns about $9.50 an hour.

The National Association of Temporary Services, the industry's trade group in Alexandria, Va., a suburb of Washington, says temp assignments can be a "bridge" to permanent work. The group says 30 percent to 40 percent of temps find regular full-time work through their temp assignments.

And some temps say the diversity of assignments helps them keep their skills up to date.

After being laid off in 1990, Holly Goodrich of Somerville, Mass., began temping as a computer graphics specialist, earning about $30,000 a year.

"Through all these jobs, I have been able to accumulate vast knowledge of various situations and software. My security is my knowledge," says Ms. Goodrich, who is 29. "In this economy, people are moving from job to job much more swiftly. I feel on the cutting edge of things."