Within a month of the assassination of Martin Luther King Jr., the FBI began investigating King's successor, looking for the same type of "immoral activities" the bureau had tried to use to discredit King, FBI files show.

The files, obtained through the Freedom of Information Act, include an April 22, 1968, memo from FBI headquarters ordering an investigation of Ralph David Abernathy. He had been King's right-hand man since the two led the Montgomery, Ala., bus boycott in 1955 that launched the civil rights movement and became president of the Southern Christian Leadership Conference after King's assassination. Abernathy died in 1990.

The memo asked the Atlanta FBI office to search its files for "background information" on Abernathy and to begin following all his activities "through established informants and sources." The investigation continued until 1974.

FBI spokesman Tron W. Brekke said the files "should be viewed in their historical context and should in no way infer that the FBI currently initiates investigations utilizing the standards of that era."

Before King's 1968 assassination, the FBI used wiretaps to gather information about his private life and extramarital affairs and had leaked it to reporters and government officials in attempts to discredit him.

The FBI files on Abernathy--previously released only to an attorney for James Earl Ray, the man who pleaded guilty to killing King--suggest the FBI under then-Director J. Edgar Hoover hoped to use similar information to discredit Abernathy after King's death.

"Little information has been developed regarding promiscuous activity on the part of Abernathy," the Atlanta FBI office told headquarters in an April 29, 1968, memo responding to what Atlanta called "the bureau's recent request for information dealing with immoral activities on the part of" Abernathy.

Atlanta noted that FBI headquarters had been sent transcripts in 1964 from a 1958 Alabama court case in which a woman accused Abernathy of having "normal and abnormal sexual relations" with her when she was 15. It also noted that Abernathy had contact with a woman in San Francisco in 1965 "that suggested a degree of affection between them."

"Our limited knowledge of Ralph Abernathy suggests he might have had some extramarital experiences," the Atlanta office concluded. "It by no means supports the conclusion his experience has been extensive or may be continuing."

A month later, Hoover had the D.C. field office check out allegations that Abernathy was "involved in illicit relations with white women" and had been "beaten by five Negroes who surprised him in bed with a white woman."

The Washington office reported that three confidential sources, as well as the local police department and the U.S. Park Police, had been unable to confirm the allegation.

In 1970, Vice President Spiro T. Agnew complained to Hoover about Abernathy's "inflammatory pronouncements." Hoover wrote that Agnew "said he thought he was going to have to start destroying Abernathy's credibility so anything I can give him would be appreciated."

The next day, the FBI sent Agnew a report that included "militant statements" by Abernathy, "information about sexual immorality, Abernathy's luxurious accommodations during the Poor People's Campaign and his support of the Black Panther Party."

When the FBI's attempts to discredit King were disclosed in a 1975 Senate investigation, Abernathy complained that he had been investigated by the FBI far more than King.

His FBI files, however, do not bear that out. While the bureau released 1,169 pages of Abernathy files, its preassassination file on King includes some 16,000 pages. There also is no evidence that Abernathy ever was the target of an FBI wiretap.

Abernathy was ostracized by many civil rights leaders before his death because he wrote in his 1989 autobiography that King had a weakness for women and that on the night before he was killed, he had sexual encounters with two women and fought with a third.

CAPTION: The FBI conducted inquiries into the private lives ofRalph David Abernathy, left, and Martin Luther King Jr.