Although endless investigations have seriously impeded the Central Intelligence Agency from carrying on its vital mission, certain CIA officials insist on continuing a "disinformation" campaign against an old enemy.

The target is Elias Demetracopoulos, a Greek expatriate living in Washington, who was best known as a relentless foe of the former Greek military dictatorship. An article in the Dec. 6 New York Times contained derogatory information about him attributed to the CIA; indeed, Times reporter David Binder told us CIA officials supplied him with information. Yet these charges are refuted by material in the CIA's own files.

Knowingly spreading false information - "disinformation" - has a long and honorable place in intelligence. But post-Watergate investigations of the CIA have made it clear that it is no longer in fashion. Thus, it is strange that the beleaguered CIA cannot let a relatively obscure foreign natural go his way in peace.

The last thing the CIA needs is a new congressional inquiry, but that is what looms in both Senate and House committees, under prodding from such CIA critics as Sen. George McGovern (D-S.D.). In a Dec. 14 letter to Daniel Inouye (D-Hawaii), chairman of the Senate Intelligence Committee, McGovern contended: "I think it is important to learn how the agency can possibly justify the selective release to the American press of materials designed to discredit Mr. Demetracopoulos or any other individual." Inouye is pushing an investigation.

Trouble between Demetracopoulos and the CIA began in 1952 when, as a Greek journalist, he feuded with then ambassador Jack Peurifoy in Athens. It climaxed when Demetracopoulos left Greece after the 1967 military revolt to lobby against the junta from Washington.

Why the CIA campaign against him was waged is explained in a Feb. 19, 1975, internal memo to then-Director William Colby. It quotes "many CIA staff officers and other informants" as believing he "has reported in a manner which injured good Greek-American relations."

Shortly afater we reported on Nov. 2, 1967, for the first time, about his lobbying against the U.S.-backed junta, we received a telephone call from a reputable CIA officer. Speaking "off the record," he warned us to beware of Demetracopoulos. CIA files showed, he said, that Demetracopoulos had been a double agent in Greece for the Soviet KGB and Western intelligence services.

But the CIA's own files branded this a lie. That same 1975 memo to Colby put it bluntly: "While he has been an annoyance, there are no hard facts in the record to show that he has worked for any foreign government against the interests of Greece (or for that matter the United States), that he is in the pay of an national government, that he has ever been a member of a foreign intelligence service."

Nevertheless, four years earlier the CIA helped prepare a scurrilous white paper against Demetracopoulos, for which the State Department later apologized. On Jan. 22, 1976, writers Russell Warren Howe and Sarah Hays Trott had a CIA-prepared memo containing derogatory information about him when they interviewed Demetracopoulos for their book, "The Power Peddlers."

"In hindsight," said the 1975 memo to Colby, "the agency may have overreacted to the provocations of Dimitrakopoulos [sic ]." But on Aug. 8, 1977, CIA Director Stansfield Turner wrote a categorical denial to Rep. Don Edwards (D-Calif.) that the CIA had disseminated false information about Demetracopoulos. Next came Binder's Dec. 6 article in The Times.

Binder wrote that "CIA records show" Demetracopoulos offered his services to the agency in 1951 - an assertion totally denied by the Greek expatriate. "The CIA records further allege," Binder continued, "that in the 1950s he was associated with both Yugoslav and Israeli intelligence services" - charges specifically refuted in the 1975 memo to Colby.

Binder also quoted a "CIA official" as saying KYP, the Greek intelligence service, at CIA's request "found no evidence of underground resistance activities" by Demetracopoulos in World War II. Yet a June 23, 1952, internal CIA memo says he suffered "imprisonment and death sentence by the Germans in World War II for sabotage activities."

Binder told us he received CIA material on Demetracopoulos years ago in Athens, but in reporting on him this fall, he was given new, unspecified information by CIA officials "past and present." With the serious business of rebuilding the nation's intelligence services at stake, Turner's CIA cannot afford time off to harass an insignificant expatriate for sins the CIA itself admits were not committed.