The White House has declassified for publication the depositions of 275 witnesses questioned last year in secret sessions by the House-Senate Iran-contra committees but has refused to release closed-door testimony of a dozen individuals, including Lewis Tambs, former U.S. ambassador to Costa Rica, according to congressional sources.

Publication is to begin this month on the first of an estimated 40,000 pages of Iran-contra depositions, chronologies and documents from the congressional investigation, the sources said.

It is expected to take several months to release all 28 volumes of testimony, four volumes of documents and two separate day-by-day chronologies that the committees prepared.

House and Senate aides who have been managing the declassification process accepted a White House hold on secret testimony of Central Intelligence Agency agents but questioned the decision on Tambs, who later testified in a public session, these sources said.

Tambs was "outspoken and irreverent" about activities and people in the Reagan administration during much of the closed-door questioning not being released, according to a source who recently read portions of the deposition.

An administration official familiar with the declassification process, who asked not to be identified, said yesterday that no depositions were being withheld for anything other than national-security reasons.

"The interagency committee applied general standards throughout," this official said, and determined that "some of the depositions are completely classified."

The official said, "I don't think I should get into specifics," but added that the declassification committee also eliminated "matters of a privacy nature."

Saying the White House "made an effort to get all the information possible out," the official said the administration officials "did not want to be accused of withholding information."

"We were not about to permit criticism that we held back politically sensitive facts," the official said.

However, congressional aides said yesterday that there has been almost no opportunity to question the White House decisions on what to delete. More than 200 cleared transcripts were delivered Jan. 29 to Capitol Hill, where only the House panel has maintained a staff of four people.

In the closed-door testimony of former White House chief of staff Donald T. Regan, released by the Senate committee last week, the White House blacked out references to First Lady Nancy Reagan in a section where Regan discussed his well-publicized conversations with her before he was fired.

"Did you speak to {words deleted} that evening?" Regan was asked. "Yes," he replied.

"And what did {words deleted} tell you?"

The entire answer was eliminated, but Regan's later response showed that the discussion involved firing people at the White House.

"It would have to be a housecleaning of people that had let {words deleted} down," Regan said, adding, "I seem to have the impression that mine was one of the heads that would have to roll," an obvious reference to a published statement by Nancy Reagan.

Regan answered the same questions in his public appearance, using Nancy Reagan's name and some of the material blacked out in the released deposition.

The administration official said that deletion of Nancy Reagan was "unique" and that "we may have screwed up" in doing it.

In another instance, the White House panel deleted all references to a covert, interagency hostage-release operation run by former White House aide Lt. Col. Oliver L. North that occurred partly in Canada.

A reference to the same operation was left untouched in a document released during confirmation hearings for CIA Director William H. Webster.

The administration official said the clearance panel deleted references to foreign intelligence services.

"We may have been overinclusive on less-important documents," the official said, "because we paid most attention to the most important documents in an effort to get all possible information out."