Herbert Romerstein, the U.S. Information Agency's specialist in detecting documents forged by Soviet intelligence agencies, has a special interest in his latest case. It involves him.

USIA officials say they have uncovered a plot to circulate a phony letter purportedly written by Romerstein.

Copies of the document were sent anonymously to The Washington Post and other news organizations. On its face, it appears to be a letter by Romerstein to Sen. David F. Durenberger (R-Minn.), chairman of the Senate Select Committee on Intelligence, last April 29 inthe aftermath of the Chernobyl nuclear power plant disaster.

Asserting that the accident could be used by the United States for "propaganda purposes," the letter advocated encouraging "our associates in European information media" to spread reports alleging that the disaster had claimed 2,000 to 3,000 victims, caused a mass evacuation from a 100-mile zone around Chernobyl and threatened Europe with widespread air and ground contamination.

Some of these assertions did appear in subsequent news reports on Chernobyl. But USIA officials insist that the reports stemmed from the confusion and rumor that swept Europe in the days immediately after the disaster. They insist that USIA made no effort to encourage the rumors and that neither Romerstein nor anyone else in USIA advocated such an idea to Durenberger or to anyone else.

Instead, the officials added, the alleged letter appears to be one of about 25 documents that pop up each year with information that seemingly implicates the U.S. government in questionable activities against other countries. Some purport to be diplomatic cables or internal memoranda, and some supposedly are letters written by officials ranging from obscure bureaucrats to President Reagan.

Some are the work of practical jokers. But most are fabricated by agents of the Soviet Union or other unfriendly governments, the officials said.

Beginning in the Carter administration, the U.S. government has made efforts to track down and expose such forgeries. The Reagan administration, with its hard-line approach to communism, has intensified the campaign by forming an interagency working group.

For the past three years, Romerstein, a former congressional staffer with a reputation as an outspoken anticommunist, has headed USIA's contribution to this effort.

He said that markings from the letter he allegedly sent to Durenberger appear to indicate that the letterhead and the signature block were taken from a letter he sent several months ago to Lt. Gen. Robert Schweitzer, head of the Inter-American Defense Board.

"It was an unclassified letter so it wouldn't have been difficult to obtain a copy," he said. "It appears that they took the heading with the USIA logotype and the signature part, cut them out and used them as the framework for a forgery."

Nancy Ness, Durenberger's communications secretary, said a check of the senator's office records did not indicate that such a letter had been received in late April. But she added, "At that time, we were receiving about 5,000 letters a day about the tax bill -- much of which is still piled up unread."

There is another reason why Durenberger's office may never have received the document even if it had been sent to him: It was addressed to the wrong office.