Anyone who has bought a plane ticket since 1987 could be party to a class-action suit alleging major airlines used a computerized fare system to fix prices.

Plaintiffs are suing USAir, Northwest, Delta, Continental, United, TWA and American airlines, alleging they used a Washington-based computer clearinghouse, Airline Tariff Publishing Co., to signal fare changes, agree on price increases and threaten fare cuts. ATP also is named as a defendant in the suit.The defendants deny the allegations and likely will oppose the class-action certification, said Delta attorney Edward B. Krugman.

If U.S. District Judge Marvin H. Shoob certifies the suit as a class action, every person who has bought a plane ticket since 1987 could technically be a plaintiff and potentially benefit from the decision, said W. Pitts Carr, the Atlanta attorney who filed the first suit in July. Shoob likely will not decide on the class-action issue until early next year.

"In 1988, 150 million tickets were sold so that would give you a rough idea of the possible magnitude of this case," Carr said. "There are literally tens of millions of people or companies who are potential class members."

A panel of federal judges placed 31 separate suits in Shoob's court Friday after deciding Atlanta was easily accessible to the most parties.

"Delta denies totally the allegations . . . concerning our participation in any conspiracy to fix prices," Krugman said.

The suit alleges the network allows airlines to respond to price changes before fares are even offered to the public. Sometimes, an airline signals its displeasure in matching a low fare by publishing it on the computer with a coded prefix, the suit says.

The suit asks for undetermined damages that would be tripled under federal antitrust laws.

ATP collects fare data from 270 foreign and domestic carriers, and transmits it to consumers, including 36,000 travel agencies, said Terrence B. Adamson, one of its Atlanta attorneys. The company handles about 160,000 fare changes a day.

Adamson said the computer network simply makes fares more competitive. However, the lawsuit's claims are similar to allegations that federal antitrust attorneys have been investigating, and Shoob has ordered that the airlines produce copies of documents turned over to the Department of Justice and not destroy other documents.