A federal magistrate ruled yesterday that evidence obtained from Michael Vernon Townley, convicted of plotting the 1976 assassination of former Chilean ambassador Orlando Letelier, cannot be used in efforts to extradite him to Argentina to face separate murder charges there.

U.S. Magistrate W. Harris Grimsley in Alexandria gave federal prosecutors until 2 p.m. Monday to present other evidence to support their claim there is reason to believe Townley had a part in the car-bomb murder of Chilean General Carlos Prats and his wife in Argentina in 1974 and should be sent there.

The ruling was seen as an important victory for Townley, whose lawyer has argued that the U.S. and Argentine governments' case for extradition rests primarily on information connected with the Letelier killing that Townley disclosed to prosecutors under a 1978 plea bargaining agreement that granted him immunity from further prosecution.

In their extradition request, the Argentines cited the similarities between the Letelier and Prats slayings and the fact that Townley's passport showed that he had been in Argentina and left around the time of Prats' death.

The U.S. attorney's office in Alexandria is representing Argentina under a 1972 extradition treaty. Lawyers close to the case said that if Grimsley rejects the prosecutor's evidence Monday, the government will have no appeal and Townley could go free.

Dressed in a dark blue suit and looking fit, Townley sat calmly through yesterday's more than five hours of court proceedings, joining in laughter at jokes from the bench and occasionally twirling a heavy moustache.

In 1978, Townley pleaded guilty to conspiracy in the murder of Letelier, who died with a coworker Ronnie Moffit in 1976 when a bomb exploded in their car as it rounded Sheridan Circle on Embassy Row.

Townley, a former Chilean intelligence operative, provided most of the U.S. government's information on the killing. In return, the government agreed in writing that it would not prosecute him for "any further crimes it may become aware of" that took place before the agreement.

He was paroled in April after serving 62 months of a 10-year sentence but, before he left prison, was arrested pending resolution of Argentina's extradition request.

In yesterday's hearing, Townley's attorney Jeffrey Johnson called to the witness stand current and former federal prosecutors who had negotiated the plea agreement in 1978, as well as one of Townley's attorneys at the time, in an attempt to show that the government was breaking the plea agreement in pursuing Townley's extradition.

Johnson said Argentina's case against Townley is based largely on evidence the federal government turned over to two Argentinian prosecutors who visited Washington in 1979. He also said that federal prosecutors, in their arguments yesterday, were citing facts Townley had divulged under the immunity agreement.

"The United States promised this man time and time again that it wouldn't use this information against him, ever," Johnson said.

Assistant U.S. Attorney Leonie Brinkema argued the plea agreement did not specifically rule out extradition and that federal law does not permit federal prosecutors to grant such a provision anyway.

"It's clear from the language of the plea agreement that international extradition was never contemplated," she said. ". . . Mr. Townley is asking the treaty to give him a totally unbargained-for benefit." Brinkema also argued that the information that the Argentinian prosecutors got from the U.S. was available from other sources.

Extradition proceedings against Townley have been linked in press reports here and in Argentina to the recent arrest in Buenos Aires of a former Bolivian interior minister, Luis Arce Gomez. He is wanted by U.S. authorities for allegedly overseeing cocaine smuggling operations.