As a budding politician in Hollywood`s acting community after World War II, Ronald Reagan was a confidential informant for the FBI, according to records released by the bureau.

The FBI documents, obtained by the San Jose Mercury News in a freedom-of- information request, show that Reagan, identified as ''T-10,'' told agents about pro-Communist influences in the Screen Actors Guild and other Hollywood organizations.

The reports show that he and his first wife, actress Jane Wyman, gave the FBI the names of actors who they believed were members of a clique with a pro- Communist line.

Rusty Brashear, a White House spokesman, said FBI officials had told him that Reagan`s involvement with the bureau was minor.

''Apparently, it was little more than those people who had been contacted by HUAC (the House Un-American Activities Committee). I`m not sure that this reference to confidential informant is quite what it sounds like.''

The documents also show that Reagan, then guild president, disagreed with tactics of the House Un-American Activities Committee in trying to rid the movie industry of Communists.

The documents show that in one FBI interview, Reagan criticized attempts by a panel of producers and actors to fire Communists from film work.

''Do they expect us to constitute ourselves as a little FBI of our own and determine just who is a Commie and who isn`t?'' he asked an FBI agent.

Reagan`s FBI file also describes his brief involvement with the American Veterans Committee and the Hollywood Independent Citizens Committee of the Arts, Sciences and Professions, which the FBI considered Communist-front organizations.

The reports suggest that Reagan, who was then spending as much time on union activity as acting, quit both groups because of his distrust of Communists.

The FBI usually does not release details about confidential informants. But Reagan`s file, released after nearly 40 years, indicates that the President was 1 of at least 18 informants the FBI used to gauge Communist infiltration of the film industry.

An FBI press officer, William Carter, said Reagan`s public status permitted the bureau to release more information about him than about private citizens.

In early February, the FBI released a 28-page file on Reagan that described his involvement with the Committee for a Democratic Far Eastern Policy, one of several groups branded a Communist front by the U.S. attorney general.

The recently released documents, 155 pages that include dozens of newspaper clippings and pages of congressional testimony, show that Reagan was an FBI informant.

The documents first mention Reagan on Sept. 17, 1941, when a Washington-based FBI agent, whose name was crossed out, wrote a memorandum to Hugh Clegg, then assistant special agent in charge of the Los Angeles division.

''As a result of my previous position as (deleted), I became intimately acquainted with the following persons who might be of some assistance to the bureau,'' the memo says.

The writer said he would be ''glad to contact'' the people and give them the name of the special agent in charge. The only name not crossed out on the list was ''Ronald Reagan, Warner Brothers studio, Hollywood.''

The first record of an FBI agent interviewing Reagan was Nov. 18, 1943, in connection with an investigation of an unidentified German sympathizer. Reagan, then 32, was assigned to the Army Air Corps` motion picture unit at Camp Roach in Culver City, Calif.

According to the FBI report, Reagan told of nearly ''coming to blows''

with the German sympathizer who made several anti-Semitic remarks at a cocktail party.

''Due to the nature of the remarks made by subject, Reagan became highly incensed and withdrew from the conversation . . . although he emphasized that considerable drinking had been done by all persons involved,'' the FBI report says.

The next record of a Reagan interview was April 10, 1947, in connection with the FBI`s investigation of the Hollywood Independent Citizens Committee of the Arts, Sciences and Professions, or HICCASP.

Before quitting the committee, Reagan attended a meeting July 11, 1946, that was under FBI surveillance.

An FBI summary of the interview with Reagan and Wyman gives some insight into Reagan`s developing anti-communism--and political--instincts.

''Reagan stated that he was present and that this meeting was precipitated by the fact he and nine other members of (the committee) had attempted to create an issue to justify their resignations,'' the FBI report says.

The report says Reagan`s group advocated a resolution condemning communism and fascism, a proposal that faced heavy opposition.

''Reagan advised, considerable discussion pro and con ensued and finally

(deleted) took the floor and stated positively and somewhat prolongedly,

`Regardless of how much discussion occurs here, I can tell you this much for certain: HICCASP will never pass any resolution condemning communism or condoning capitalism.` ''

The resolution lost 60-10 at a later meeting, the FBI report says, and Reagan resigned that night by telegram.

The longest excerpt in the FBI documents dealt with Reagan`s activities in the Screen Actors Guild and his appearance before the House Un-American Activities Committee in October, 1947.

In the April 10, 1947, interview in which he described the Hollywood Independent Citizens meeting, Reagan also outlined the workings of the Screen Actors Guild, the documents show. He was elected guild president that year.

According to the report, Reagan and Wyman told the FBI that they had noticed two cliques in the guild that ''follow the Communist Party line'' on all policy questions.

The FBI report lists actors and actresses named by Reagan and Wyman. Their names were deleted, but the report shows that the Los Angeles FBI cross- referenced the names to determine who were known Communist Party members.

On Dec. 19, 1947, Reagan was identified explicitly as confidential informant ''T-10'' on the 40th page of a bureau report, ''Communist Infiltration of the Motion Picture Industry.''

Though the names of the 17 other informants listed were blacked out, the report says that ''T-9'' was a ''well-known actress.'' The report quotes her as saying that Reagan had ''seen the light'' and was sincere in his efforts to keep radical guild members out of power.

Other documents depict Reagan, then a Democrat, as disturbed by some government tactics in trying to root Communists out of the film industry.