Prosecutors spelled out plans for Pettitte in court papers Friday outlining evidence the government intends to introduce that shows many major league baseball players used performance enhancing drugs.
The government says Pettitte, who recently emerged from retirement to sign a New York Yankee minor league contract, will testify about using human growth hormone with the assistance of Brian McNamee, Clemens' longtime trainer and chief accuser.
The prosecutors also plan to cite former Sen. George Mitchell's investigation that found illegal use of performance-enhancing substances by more than 80 major league players. Clemens denies lying when he told Congress he didn't use them.
Jury selection begins April 16.