The Department of Energy may have improperly declassified "a limited number" of documents that would be of "extremely great interest to foreign nations seeking to build a hydrogen bomb," an informed source said last night.

The possibility is to be explored at a hearing today by the Senate subcommittee on energy, nuclear proliferation and federal services. If it proves out, the sources said, it would mean that the documents were accessible to persons using the public library of the Los Alamos Scientific Laboratory in New Mexico.

Originally, the subcommittee knew of and was investigating a single declassification error. It involved only one document, UCRL 4725, which, DOE now says, gives "some precise details" on the devices that trigger H-bombs and contains data "of value to someone who is interested in designing a bomb."

The paper was originally classified "secret," and should have stayed in that status, DOE's nuclear weapons chief said last week. Terming the security breach "Serious" even if "not catastrophic," Maj. Gen. Joseph K. Bratton said, "We have egg on our face."

For nearly four years, UCRL 4725 was on the library's open shelves. Two weeks ago, however, DOE's attention was called to it when it was examined and copied by Dimitri A. Rotow, an American Civil Liberties Union researcher.

The ACLU represents editors of the Progressive magazine, which is trying to overturn a court order, obtained by the government, restraining publication of an article on the bomb.

In today's hearing, it was learned, Princeton University's Theodore B. Taylor, a former designer of nuclear weapons at Los Alamos, is to testify that the accidental declassification of UCRL 4725 may be a cause for graver concern than heretofore has been acknowledged.

DOE has attributed the declassification to a clerical error. But the subcommittee now has indication that the problem wasn't merely one involving a slip-up by some low-level clerk, but one possibly involving a series of administrative breakdowns resulting in multiple erroneous declassifications.

Subcommittee Chairman John H. Glenn Jr. (D-Ohio) has said that the declassification of UCRL 4725, by itself, was shocking, because the paper contained information that could help another country "take years off the time needed to develop a hydrogen bomb."

In hearings more than a year ago, DOE had assured Glenn that it was undertaking a thorough review of DOE libraries to make sure that they contained no documents that should be classified.

In a related development, the ACLU's Washington office released a list of 42 documents that researcher Rotow copied at the Los Alamos library before DOE closed the public shelves for a security review.

ACLU lawyer Avram Soifer told reporters that Rotow believes the documents as a group would be more helpful than UCRL 4725 to a foreign country wishing to build a thermonuclear weapon. Last night, a DOE spokesman who obtained the list said it is being studied, but that he had no comment.

No one suggested that the formerly classified documents among the 42 papers had been improperly declassified. Rotow, who says he found them within hours of starting work in the library May 7, is to turn them over to the subcommittee today.

Soifer said that a protective order issued by a federal judge in the Progressive case prevented the ACLU "from making them public without first submitting them to the government for censorship."

Morton Halperin, director of the ACLU Project on National Security and a specialist in nuclear proliferation, termed the protective order "ineffective, in that the information is not secret." CAPTION: Picture, ACLU lawyer Avram Soifer displays an unclassified document at news conference. By Fred Sweets - The Washington Post