The FBI inflitrated the American Civil Liberties Union when it was formed in 1920 and kept files on leading members, including Felix Frankfurter, Helen Keller, Jane Addams Upton Sinclair and Clarence Darrow.

An FBI dossier on Frankfurter, prepared when he was a Harvard law professor before he became a Supreme Court justice, said he was "considered a dangerous man by United States government employees."

Helen Keller, the famed blind and deaf author and lecturer, was described in the files as a "writer on radical subjects."

The surveillance is disclosed in 3,072 pages of FBI documents turned over to the ACLU this month under a federal court order.

U.S. District Court Judge Hubert Will, acting in a freedom of information suit filed by ACLU officials here, ordered the FBI to release about one-fifth of its files on the ACLU each month untill all 20,000 pages have been turned over.

The first documents span 1920 through 1942. About 17,000 pages amassed from 1943 on are to be released over the next four months.

The FBI declined to comment on th documents and refused to say whether the ACLU surveillance is continuing, citing FBI policy not to confirm or deny that an investigation is being conducted.

The first documents released show that the FBI infiltration began in January, 1920, when the ACLU was formed as an extension of the Civil Liberties Bureau, organized by Roger N. Baldwin during World War I to defend war dissenters.

This is how Baldwin explained the ACLU philosophy in 1920: "We stand on the general principle that all thought on matters of public concern should be freely expressed without interference . . . The principle of freedom of speech, press and assembly, embodied in our constitutional law, must be constantly reasserted and applied to be made effective."

Over the years, the ACLU has asserted Baldwin's principles in courts across the country, defending Communists, Nazis and klansmen without regard to the unpopularity of the clients' causes.

FBI "confidential operatives" infiltrated the ACLU at every level, providing detailed reports on closed meetings, membership lists and financial contributions and, in some cases, copies of private correspondence between ACLU officials and other private citizens, the documents show.

The files disclosed that J. Edgar Hoover, who became FBI director in 1924, personally supervised the surveillance, down to approving a $5 expenditure to take out an ACLU membership in a fictitious name.

Despite his firsthand involvement, however, Hoover repeatedly denied to Baldwin that the organization was under investigation. And, although the ACLU was described in the files as "nothing more than a front for the Communists," Hoover also denied to Baldwin that the ACLU was suspected of being subversive.

From the beginning, persons affiliated with the ACLU were described in the files as "radical." "Communists," "Socialists," "Russians" or "Jews." Wealthy backers were characterized as "parlor Bolsheviki" or "parlor pinks."

In 1922, the FBI prepared a dossier on each member of the ACLU's National Committee, including Frankfurter, who, in addition to his duties at Harvard, was teaching a course titled "The Law Enforcement Officer and the Prosecutor" at the FBi National Academy in Washington.

Despite his connection with the bureau, Frankfurter is accused in his FBI dossier of delivering "radical lectures" to a sociology class at Radcliffe. "He is also attorney for the Communist party at Boston and other radical organizations and is considered a dangerous man by United States government employees," the dossier continued.

Nobel laureate Jane Addams, founder of Hull House, the famous settlement house in Chicago, was viewed by the FBI as a "zealous and consistent supporter of radical and revolutionary movements."

Upton Sinclair, the novelist, was allegedly "working to build up a fat hidden bank account in Toronto, Canada," and an FBI agant who monitored a meeting where Siclair appeared reported to Hoover: "Upton did not talk very long but ridiculed religion and roasted the Pope."

William Allen White, the nationally renowned 1930s author and journalist from Emporia, Kan, was a "member of the Committee on Militarism in Education which is against military training in American schools . . . a member of the Foreign Policy Assn. which is a radical organization affiliated with the American Civil Liberties Union . . ."

At least three Nobel laureates, several congressmen, two future Supreme Court justices and scores of authors, journalists, actors, professors and clergymen all ended up in FBI files. A sampling:

Emily Blach, Stephen Vincent Benet, Van Wyck Brooks, Heywood Broun, Pearl Buck, Eugene V. Debs, John Dewey, John Dos Passos, Melvyn Douglas, Theodore Dreiser, Norman Hapgood, Robert Morse Lovett, Thurgood Marshall, John P. Marquand, Jeanette Rankin, Elmer Rice, Dr. Raymond Gram Swing, Norman John A. Ryan, Robert E. Sherwood, Thomas and Harry P. Ward.

On Feb. 26, 1929, Hoover asked the New York FBI office to make a "confidential undercover inquiry" to see if ACLU files contained a certain pamphlet. Next day agent C. D. McKean wrote Hoover: "A search of the files (at ACLU headquarters) from Jan. 5, 1928, to Feb. 21, 1929, failed to develop any such circular as the one described by you in which white slave and narcotic violations are described as 'minor offenses./"

In 1941, James G. Findlay, an agent in New York, sent this observation to Hoover and to regional FBI offices: The ACLU's alleged purposes are to fight for the preservation of the civil rights of minorities, but it is made up largely of Communists, Socialists and radicals of every degree, and its activities clearly show that it is nothing more than a front for the Communists, and that approximately 90 per cent of its time and money are devoted to the defense of communism.

The files contain a transcript of a May 21, 1943, telephone conversation between Baldwin and A1 Goodman, a Socialist Workers Party leader whom the ACLU was defending in first amendment cases. Nothing indicates whether the tap was on Baldwin's or Goodman's telephone, but either would have been illegal under the Federal Communications Act of 1934, which flatly banned wiretapping.

In 1940 a New York agent wrote to ask Hoover whether he should take out a $5 annual membership in the ACLU or merely subscribe to the ACLU reports at $3 a year. "The Bureau will approve the obtaining of membership condidentially . . . at a cost of $5 annually," Hoover replied.

On Oct. 27, 1942, Baldwin wrote Hoover to complain that "some of your agents seem to have some notion that the Civil Liberties Union is a subversive organization and that connection with it justifies investigation."

Hoover wrote back: "I can assure you that there are no outstanding instructions from this bureau which would either directly or indirectly lead any special agent . . . to assume that connection with the American Civil Liberites Union is presumptive indication of subversive activity and therefore necessitating an investigation."

The following month, Baldwin wrote to Hoover suggesting that ACLU literature be distributed to FBI field offices to acquaint agents with the ACLU's activities and purposes. Hoover rejected the suggestion, informing Baldwin on Nov. 7, 1942:

"It has been the consistent policy of this bureau in the conduct of its investigations to seek from either individuals or organizations information which may be available. But this information is not sought until or unless there are substantive grounds for conducting investigation of such individuals or organizations. I can assure you that should the occasion ever arise when this bureau is desirous of obtaining any information concerning the activities of the American Civil Liberties Union, I will not hesitate to communicate with you in the first instance."

When Hoover wrote that, the FBI had infiltrated and spied on the ACLU for 22 years and had a 3,000-page file on its activities and members.

Baldwin, now 93, said in an interview last week that he probably will never understand why the FBI went to so much trouble and expense to infiltrate the ACLU. "I think it is aterrible waste of effort and government funds to spy on an organization as open as the ACLU," he said. "It just illustrates the stupidity, of the police agency that operates on the grounds of suspicion instead of fact."

Baldwin added: "They didn't have to spy to get information. The agents collected things that were on the public record. We would have been glad to give it to them. We didn't have anything to conceal."

When informed that the FBI had described Frankfurter as considered dangerous by federal employees, Baldwin replied: "Mr. Frankfurter was just about as dangerous as the Bill of Rights."

Baldwin said the FBI's infiltration "amounts to secret police tactics.The great error of the FBI is that they investigated people's political opinion."