Carney argues that Stearns has a conflict of interest because he worked as a prosecutor in the U.S. attorney's office in Boston at the same time as O'Sullivan. The 1st U.S. Circuit Court of Appeals has not indicated when it will rule on the request.
In his ruling, Stearns agreed with prosecutors that he should rule on Bulger's immunity claim before trial.
"Resolution of defendant's immunity claim requires an inquiry only into the existence, scope, and validity of the alleged agreement he made with New England Organized Crime Strike Force Chief Jeremiah O'Sullivan," Stearns wrote.
"It does not require the presentation of evidence regarding the commission of any of the nineteen murders or other crimes with which defendant is charged. The government's motion is thus not only appropriately raised, but calls for pretrial resolution."
Stearns noted that Bulger has the right to testify in his own defense at trial, regardless of the court's ruling on his immunity claim. But he also noted that testifying in one's own defense can be limited by rules relating to "materiality and perjury."
Stearns said there has been a "paucity of information" provided by Bulger's defense regarding his claim of immunity. Bulger's lawyers have said only that O'Sullivan — some time before December 1984 — promised Bulger immunity from prosecution for all past and future crimes, up to and including murder.
A spokeswoman for U.S. Attorney Carmen Ortiz declined to comment on the ruling.
Bulger's lawyers have argued that if the judge prevents Bulger from presenting a defense based on his immunity claim, it will deprive Bulger of his right to a fair trial.
"Whether or not the defendant was granted immunity is at the core of this case," the defense wrote in a court filing Friday.
Carney recently stunned prosecutors when he said Bulger was never an FBI informant. In documents filed in court Friday, prosecutors said the claim that Bulger was not an FBI informant, but still had immunity is "both strange and unsubstantiated."
In their statement Monday, Bulger's lawyers said Bulger is prepared to testify "how and why he received immunity" from O'Sullivan "for past and ongoing crimes."