The jury in the trial of James "Whitey" Bulger has begun deliberating whether he is guilty of murder and racketeering charges that could put him in prison for the rest of his life.

For almost two months, witnesses have testified about 19 murders the government says the 83-year-old defendant committed or ordered in the 1970s and 1980s.

A sweeping 32-count indictment also charges Bulger with drug dealing, extortion and maintaining an arsenal of guns that prosecutors described as the "tools of the trade."

Bulger has pleaded not guilty to all charges, though his lawyers admitted on the first day of the trial that Bulger had been a drug dealer, extortionist and loan shark.

Before sending the jurors to deliberate, US Distrct Judge Denise Casper reminded them they could not consider Bulger's decision on Friday not to testify in his own defence at a trial he called "a sham."

"No inference of guilt or of anything else may be drawn form the fact that the defendant did not testify," she said.

Twice during the trial, the judge admonished Bulger for cursing at witnesses who said he was an informant for the FBI, which ignored his crimes in return for tips about other criminal gangs.

Bulger insisted he was not "a rat," but conceded that he did pay FBI agents for information he could use.

The jury heard former gangsters, extortion victims and law enforcement officials recount the gory details of killings they said  Bulger either ordered or carried out.

The murders included innocent people gunned down in botched attempts to kill rivals and fellow criminals the gang killed because Bulger suspected they were talking to police.

Former mob lieutenant Stephen "The Rifleman" Flemmi testified that Bulger strangled two women, Flemmi's girlfriend and step-daughter, because the mob boss feared they knew too much.

For most of the trial, Bulger, who has lost the shock of light hair that earned him the nickname "Whitey" in his youth, sat quietly at the defense table, writing on a yellow legal notepad.

Bulger fled Boston in 1994 after a tip from a corrupt FBI agent that arrest was imminent.

He lived in hiding for 16 years, many of them listed on the bureau's Ten Most Wanted list.

Agents caught him living in a seaside Santa Monica, California apartment in June 2011.