Bustos' corner threw in the towel 2 minutes, 26 seconds into the sixth round of the scheduled eight-round welterweight fight.

Spadafora, who hadn't fought in his Pittsburgh hometown since 2004 because of legal problems, improved his career record to 42-0-1.

The bout was the 33-year-old Spadafora's first with Hall of Fame boxer Pernell Whitaker as his trainer.

Bustos, a counterpuncher from Argentina who had trouble landing blows against Spadafora, is 25-13-3.