WBUR 90.9 via AP

Reputed Boston mob boss James "Whitey" Bulger.

A judge is set to hear arguments on reputed Boston gangster James "Whitey" Bulger's claim that he was given immunity to commit crimes while he was an FBI informant.

Bulger, the former leader of the Winter Hill Gang, is awaiting trial in a sweeping racketeering indictment accusing him of participating in 19 murders. He claims a federal prosecutor gave him immunity for his crimes while he was providing the FBI information on local leaders of the Mafia, his gang's main rivals.

Bulger's lawyers want to use his immunity claim as a defense at his trial. Bulger contends former federal prosecutor Jeremiah O'Sullivan, who led the New England Organized Crime Strike Force, gave him immunity while he was an FBI informant in the 1970s and '80s. O'Sullivan, who died in 2009, denied ever making an immunity deal with Bulger during his testimony before Congress in 2002.

Federal prosecutors have called Bulger's claim "absurd" and have asked U.S. District Judge Richard Stearns to decide the issue before trial.

Stearns has scheduled a hearing Wednesday.

Bulger, 83, fled Boston in 1994 and was one of the FBI's most wanted fugitives until his capture in Santa Monica, Calif., in 2011.

-The Associated Press