One of the nation's two major builders of transit buses said yesterday that it will not bid on a contract to manufacture 503 new buses that, by federal regulation, are the only kind transit authorities can buy with federal aid after September.

The announcement yesterday by Grumman Flxible makes it probable that no American firm will seek to sell the controversial new vehicle, called Transbus. Transbus is mandated to have low floors and ramps to make it fully accessible for wheelchair occupants.

General Motors Corp., the other American builder, has been rumored for months to be out fo the running in the Transbus stakes. GM spokesman Frank Ferone said yesterday, "We're still in the process of studying those specifications and in all honesty it does not appear likely that we will bid."

Bid-opening on the first Transbus order has been postponed from this month to May. Several foreign manufacturers have expressed some interest in Transbus, but it is not known if they will bid. Federal transit legislation includes a strong "Buy American" clause, but a provision would permit Transportation Secretary Brock Adams to waive that clause.

Adams has been a strong supporter of the Transbus program and has insisted that the specifications were within the capability of American manufacturers. He said yesterday that "I am very disappointed that [Grumman Flxible] has said they do not intend to bid. I hope that General Motors, with its multimillion-dollar operation, will help us carry out the promise to make an accessible bus."

Adams said it "would be a shame if we have to go abroad." If nobody bids, he said, "Then I will have to take a look at the program. But I'm not going to falter at this point."

Thomas J. Bernard, Grumman Flxible's president, said that "we made what we think was a good-faith effort to try to be responsible." However, he said, development of Transbus was a "tremendous technological risk"; that the "terms and conditions of the contract presented an onerous business risk," and that existing buses already solve "80 percent" of the accessibility problem.

Both Flxible and GM are now selling new-look buses for about $105,000 to $120,000 each. Both have contended that those buses meet most of the requirements of full accessibility plus use proven technology. Proponents of Transbus have charged that GM and Flxible have attempted to torpedo the Transbus program so they could recover development costs on their newlook buses.

Bernard said yesterday that Flxible estimated roughly that each Transbus would cost about $230,000 -- about twice the cost of a new bus today.

The Department of Transportation, he said, "has been seeking a more productive bus. We believe that a bus that weighs more, gets fewer miles per gallon, has fewer seats and less standing room is not a more productive bus."

The specifications for Transbus were drawn after three manufacturers built prototypes and after years of hearings and debate. Transbus has been vigorously supported by the increasingly well-organized lobby for the handicapped.

When the Urban Mass Transportation Administration, one of Adams' agencies, mandated Transbus last September, it agreed that the September 1979 deadline could be changed if it proved unrealistic. UMTA grants provide 80 percent of the purchase price for new buses.Local authorities pay the other 20 percent.