The D.C. Court of Appeals ruled yesterday that U.S. prosecutors may use previously suppressed evidence against a Washington police officer accused of assaulting a narcotics suspect in 1978 and then attempting to cover up the incident.

Judge Catherine B. Kelly's ruling in effect opens the door for 3rd District officer Elwood C. Anderson's long-delayed trial to proceed in Superior Court.

Anderson was indicted in 1979 on charges of assault and obstruction of justice after another officer, Leonard Campbell, reported witnessing Anderson kick a handcuffed narcotics suspect who had tried to escape.

Campbell alleged that six other police officers had also witnessed the Sept. 12, 1978, incident in a northwest Washington alley. When questioned, all six said they did not see Anderson kick the suspect. The alleged victim also said he had not been kicked.

During an investigation of the matter, police officials granted Anderson immunity from prosecution, after which Anderson said the allegations were untrue. Within months, however, the alleged victim recanted his original account told police that Anderson had paid him to lie to police investigators about the incident.

The U.S. Attorney's office subsequently decided to go ahead with prosecution of Anderson, despite the earlier immunity grant.

Meanwhile, Campbell was ostracized and harassed by his colleagues for turning in his fellow officer and breaking the unwritten "code of silence" that exists as a mutual protection pact among officers.

"I'm glad that it's finally coming to a head now, after all this time," Campbell said after learning of the court's decision. "Maybe the truth will come out about those other officers who witnessed this and refused to state what really happened."

Anderson, who was suspended from the force without pay, pleaded innocent to the charges. The immunity grant became the focus of a legal debate when his attorney argued that the agreement should have prevented the U.S. attorney's office from using much of its evidence against Anderson.

Judge Kelly ruled that the government's case against Anderson had been obtained independently of anything Anderson told investigators after receiving immunity, and thus could be introduced at trial.

Anderson's attorney, Edwin C. Brown Jr. said, "I'm still of the firm belief that the police department did abuse the immunity agreement given to officer Anderson, contrary to the requirements of Supreme Court law." Brown said he may ask the full nine-member court to hear the matter.

Chief Judge Theodore R. Newman Jr. concurred in Kelly's 2-to-1 panel decision, which reversed a lower court ruling. Judge George R. Gallagher dissented, asserting that the government unfairly had broken its promise.