The Federal Bureau of Investigation acknowledged today that its agents plotted in 1970 to besmirch the reputation of Jean Seberg, the actress who committed suicide last week, by planting a rumor with news organizations that she was pregnant by [a] high-ranking member of the Black Panther Party. The action against Miss Seberg, part of the F.B.I.'s counterintelligence program COINTELPRO, was intended to discredit her support of the black nationalist movement. According to a document dated April 27, 1970, the Los Angeles office of the F.B.I. requested permission from J. Edgar Hoover, then Director of the bureau, to publicize Miss Seberg's pregnancy, saying it was felt the possible publication of Seberg's plight could cause her embarrassment and serve to cheapen her image with the general public." Romain Gary, the prominent French author and diplomat who was Miss Seberg's husband in 1970, said at a news conference in Paris last week that the baby was his and that the F.B.I. had destroyed the actress's life. The bureau could not say today how many celebrities or others had been harassed or otherwise adversely affected by COINTELPRO activities similar to those directed at Miss Seberg. However, the bureau's animus toward the Rev. Dr. Martin Luther King Jr. and its activities against him are well documented. As with all documents released by the F.B.I., those relating to Miss Seberg were issued with names of all other living persons deleted.