(CNN) -- James "Whitey" Bulger, the reputed former Boston mob boss who was arrested last summer in California, is planning to argue that his upcoming case should be dismissed because of an immunity agreement he had while working as an FBI informant in the 1970s, according to court documents and the defendant's attorneys.
In a motion filed with presiding Judge Richard Stearns, Bulger's attorneys said Monday that they plan to pursue an immunity hearing to clear their client of 19 murder charges.
"The defendant reached an agreement with the Department of Justice through its agent during the 1970s," attorney J.W. Carney Jr. argued in the motion. "The immunity agreement fully protects the defendant from prosecution for all of the crimes currently under indictment."
Bulger, the alleged former head of Boston's notorious Winter Hill Gang, made headlines when he was arrested in June 2011 in Santa Monica, California, after being on the run for 16 years.
Prior to his sudden departure from Boston, he cooperated as an informant with disgraced ex-FBI agent John Connolly Jr., who is currently serving a 50-year sentence for second-degree murder and racketeering.
According to an indictment against Connolly filed in 2000, Bulger became his confidential informant in the fall of 1975.
"When the United States Attorney's Office indicted the defendant for alleged past crimes, they directly violated the immunity agreement that the defendant had bargained for, had relied upon, and had been promised," Bulger's attorneys said in the motion.
But prosecutors disagree.
"Being an informant in and of itself, having certain representations by law enforcement agents, does not provide sufficient or adequate immunity," U.S. Attorney Carmen Ortiz told CNN affiliate WCVB.
Bulger is not the first member of the Winter Hill Gang to claim immunity. Stephen "The Rifleman" Flemmi, Bulger's alleged right-hand man and fellow Connolly informant, had also argued unsuccessfully for immunity from prosecution.
He later cooperated, avoiding a potential death penalty sentence, and is serving life terms without parole.