By Ian Simpson

WASHINGTON (Reuters) - New York Yankees pitcher Andy Pettitte testified on Wednesday in Roger Clemens' federal perjury trial that there was a 50-50 chance he had misheard his friend and former teammate say he used human growth hormone.

In a potential blow to the prosecution, Pettitte said he might have misunderstood Clemens when the ace pitcher told him in 1999 or 2000 he had used human growth hormone to recover from an injury.

But in 2005, Clemens told Pettitte he had been referring to his wife Debbie's use of the drug.

"As you sit here today, if you think about it in your own mind, it's 50-50. You might have heard it, or you might have misunderstood him. Is that fair?" defense attorney Mike Attanasio asked.

"I'd say that's fair," said Pettitte, who was in his second day on the witness stand.

The admission prompted lawyers for Clemens to ask that Pettitte's testimony about the critical conversation be struck, since the pitcher cannot be sure of what he heard.

Pettitte's testimony is considered crucial to the government's case. Having part of it struck - particularly his testimony about the HGH conversation with Clemens - would be a major blow to the prosecution.

Pettitte also said he had not told Clemens about being injected with human growth hormone in 2002 and injecting himself with the drug in 2004, both times to deal with injuries.

Clemens was not present at the injections, Pettitte said shortly before his testimony wrapped up at mid-morning.

John Goldman, a New York sports lawyer with Herrick, Feinstein LLP, said Pettitte's testimony on the 1999 or 2000 conversation "is not a deal-breaker" given the ebb and flow of jury trials.

Pettitte had not retracted his testimony, but only said he might have misunderstood Clemens, Goldman said.

Clemens, 49, is being tried for a second time on federal charges of lying to the House of Representatives' Committee on Oversight and Government Reform in 2008 about whether he used anabolic steroids and human growth hormone.

The panel was investigating drug use in Major League Baseball and was weighing an independent report by former Senator George Mitchell.


Clemens, a seven-time winner of the Cy Young award, baseball's highest annual honor for a pitcher, is among the biggest baseball names linked to alleged drug use.

Former Internal Revenue Service agent Jeff Novitsky, who had investigated Clemens' former personal trainer Brian McNamee as part of a probe into performance-enhancing drugs, began testimony on Wednesday afternoon.

He told jurors McNamee had turned over three plastic bags of needles, syringes, bloody swabs and other materials to him and other federal investigators in January 2008. Some of the material was stuffed inside a crumpled Miller Lite beer can.

In a setback for the defense, U.S. District Judge Reggie Walton ruled Clemens' lawyers could not touch on why McNamee had been let go by the Yankees in 2001.

McNamee left the Yankees after police investigating a Florida rape case said the trainer had lied to them. McNamee was never charged in the case.

After leaving the Yankees, McNamee continued to work with Clemens and Pettitte as a private trainer.

Walton said he had already ruled that the "nature of the underlying conduct is not admissible."

Walton also ruled he would not allow prosecution witness Steve Fehr, counsel to the Major League Baseball Players Association, to testify about his contacts with Mitchell and Clemens' agent, Randy Hendricks. Mitchell had wanted to interview Clemens for his report on drug use.

Walton said Fehr's testimony would violate attorney-client privilege and approval would be overturned on appeal.

"You're beating a dead horse," he told prosecutors.

Although Pettitte had told congressional investigators McNamee had injected him with human growth hormone, the judge blocked similar testimony at the trial on Tuesday, ruling it could be prejudicial to Clemens' defense.

Pettitte was allowed to testify that he received injections but did not say from whom.

Clemens first went on trial last July, but Walton declared a mistrial because prosecutors showed jurors a video clip that included material he had banned from the case unless it was raised by Clemens' defense team.

Pettitte has come out of retirement to bolster the Yankees' pitching staff. He is playing his way back into shape with a Yankee farm club.

(Editing by Paul Thomasch, Mohammad Zargham and Todd Eastham)