Postal unions opened negotiations with management yesterday on 1978's largest single labor contract, with the union vowing they will not be made "guinea pigs" in the administration's campaign to reduce inflation.

The bargaining on behalf of roughly 570,000 postal workers is the first to begin since President Carter last week called on labor and management to restrain cost increases. It presents a major test of the anti-inflation initiative, right in the government's own backyard.

The importance of the postal takes was underscored only two days ago when Robert S. Strauss, the President's new anti-inflation trouble-shooter, designated postal workers, teamsters and environmental regulators as the immediate targets of the inflation-cutting drive.

But both the U.S. Postal Service and the four main postal unions appeared to reject outside intervention as they gathered at the Shoreham Hotel to begin talks on a contract to succeed the three-years pact that expires July 20.

"With all due respect to our opposite members in postal management, the presence of Jimmy Carter hangs heavy over this table," said Andrews, president of the 299,000-member American Postal Workers Union, the largest of the four. "It represents a close encounter of a strange kind," he added.

James V.P. Conway, deputy postmaster general and chief negotiator for the quasi-independent postal agency, did not mention Carter, Strauss or the anti-inflation drive, but said pointedly:

"Postal management is in complete command to act for the Postal Service . . . in this collective bargaining process, irrespective of any speculation to the contrary."

The negotiations, expected to last three months, come at a time of fresh debate over the Postal Service and its costs, efficiency and quality of service.

The House recently passed a bill to restore some presidential and congressional control over the agency, which Congress made semi-autonomous in 1971 on the theory that this would make it more businesslike. The agency has cut losses but opposition by trying to close small post offices, eliminate Saturday deliveries and reduce for 85 percents of the Postal Service's costs, an expensive new contract would add to pressure for rate increases, higher subsidies or service cutbacks, or a combination of them all.

The Postal Service's maneuvering room is squeezed by administration pressures to keep down costs on one hand and the risk of a strike on the other hand.

Union leaders are playing down the possibility of a walkout, which is possible although technically banned by law. Some postal workers struck in 1970, but there have been no stop-pages since then.

Instead, they focussed on the administration's attempt to make them a model of wage restraint. "We do not gladly accept penalties for the irresponsibility of other segments of the American economy," Andrews said. "Postal workers can't be scapegoated into subsidizing the Postal Service's fiscal and technological problems" or set up as "guinea pigs" in a new effort to contain inflation, said Lonnie Johnson, director of the Mail Handlers Union.

Hanging out in front of the postal workers and others who will be bargaining this year, including railroad, construction and retail food workers, is the 37 percent, three-year increase won recently by the United Mine Workers after a 110-day strike.

The postal workers, who earn an average annual salary of about $15,000, up more than 100 percent in eight years, according to Postal Service figures, have not disclosed their wage target for this year and are stressing job security in the face of the Postal Service's drive to reduce payrolls.

The postal unions already have a no-layoff guarantee and want it broadened to bar elimination of job slots even when workers quite or retire. They are also seeking mechanization protections, a shorter work week, overtime increases and 21 holidays instead of the current 15 a year, accoring to an APWU publication.

The last postal contract in 1975 provided for a wage-and-benefit increase of 22 percent over three years, according to the Postal Service. Carter's policy calls for holding increases "significant below" the average of the previous two years.