An FBI agent admitted today that he made misleading entries about a key informer overheard by FBI eavesdroppers in a controversial organized-crime investigation known as "Operation Tumcon."

Agent Robert A. Levinson also acknowledged under cross-examination in pretrial hearings that he did not tell government prosecutors or the head of the FBI office here what he had done to mask the informer's identity.

The handling of the informer, ex-convict Michael Orlando, has become a central issue in the criminal case in Bronx Supreme Court against former labor secretary Raymond J. Donovan and nine others. The FBI's long-secret "Tumcon tapes" have emerged as the principal evidence in the case. Defense attorneys, saying the investigation was tainted, are seeking to have them suppressed.

Levinson said Orlando's voice turned up within a month on "the wire" the FBI installed Jan. 4, 1979, at the south Bronx warehouse of a reputed Genovese crime family member, William P. Masselli. Orlando had gotten close to Masselli in 1978 on the FBI's instructions, the court was told. Information supplied by Orlando and another informer, along with a Nov. 30, 1978, meat hijacking observed by FBI agents, provided the grounds for "probable cause" for the court-authorized surveillance.

Defense lawyers pointed to government records showing that Orlando was overheard on Feb. 2, 1979, discussing with Masselli the purchase of chemicals to make synthetic cocaine. He was overheard as a participant in two truck hijackings in March 1979.

Levinson was the only man on the FBI's 10-agent Tumcon squad who knew Orlando's identity. As a result, Levinson has said he was afraid to put Orlando's full name down "without it becoming obvious" later "that Orlando was an informant." Levinson said he decided to list Orlando as "Mike LNU (Last Name Unknown)" until he was identified through normal investigation.

"I would not characterize it as a false entry," Levinson said under questioning today. "It was not correct."

Levinson said he told his immediate FBI supervisor about the ploy "around the time" of an April 9, 1979, meeting with Neil J. Welch, then assistant FBI director in charge of the New York office. But he did not tell Welch.