WASHINGTON (CNN) -- Roger Clemens said Wednesday he received only vitamin shots from Brian McNamee, but the ex-trainer insisted before a House panel that every injection contained steroids or other performance enhancers.
Clemens sat mere feet from his chief accuser as the two men, both under oath, offered lawmakers starkly conflicting accounts about the injections McNamee administered years ago.
The committee's chairman, Rep. Henry Waxman, said, "Someone isn't telling the truth."
"If Mr. McNamee is lying, then he has acted inexcusably and he has made Mr. Clemens an innocent victim," Waxman said . "If Mr. Clemens isn't telling the truth, then he has acted shamefully and he has smeared Mr. McNamee. I don't think there is anything in between."
Clemens told reporters afterward that he was "very thankful and very grateful" for the chance to answer the allegations.
In a nearly direct exchange with McNamee, Clemens told the House panel that McNamee injected him with vitamin B-12 on three occasions, but never with steroids or other illegal substances.
McNamee -- who served as Clemens' trainer until 2007 -- countered that he injected Clemens with only testosterone, the steroid Winstrol or human growth hormone.
"I've never given Roger B-12," McNamee told the House Committee on Oversight and Government Reform.
Clemens' testimony to the panel examining performance-enhancing drug use in America's pastime also failed to jibe with an affidavit provided by fellow New York Yankees ace Andy Pettitte, who has told the committee that in 1999 or 2000 Clemens "told me he had taken HGH."
Saying Pettitte was and will remain a close friend, Clemens said he believes Pettitte made a mistake.
"I believe Andy has misheard, Mr. Congressman, on his comment about myself using HGH, which never happened," Clemens said.
Insisting he's never taken "shortcuts," a clearly miffed Clemens opened Wednesday's testimony saying he resented accusations that have forever tainted his legacy, which includes a record seven Cy Young Awards.
"No matter what we discuss here today, I am never going to have my name restored, but I've got to try and set the record straight," Clemens said.
McNamee flatly challenged Clemens' remarks, saying, "I injected those drugs into the body of Roger Clemens at his direction. Unfortunately, Roger has denied this and has led a full-court attack on my credibility." Watch McNamee apologize for "tainting our national pastime" »
McNamee defended his credibility and told lawmakers he was promised no special treatment "for fingering star players."
"I have no reason to lie, and every reason not to. If I do lie I will be prosecuted," he said. "All that I was ever told was to tell the truth to the best of my ability, and that is what I have done. I told the investigators that I injected three people, two of whom, I believe, confirm my account. The third is sitting at this table."
McNamee said he believes he stopped giving baseball players performance-enhancing substances in 2002. He called steroid usage "pretty prevalent" among players at the time. Watch Clemens explain how he treated McNamee »
Grilled on why he provided players with such substances, McNamee replied, "I just accepted it as the norm, and it was part of the culture in baseball."
Missing from Wednesday's hearing was Pettitte. He had been named along with Clemens and other players in ex-Sen. George Mitchell's scathing report alleging widespread steroid use in baseball.
Pettitte previously acknowledged using human growth hormone in 2002. The House panel revealed Wednesday that Pettitte admitted using it again in 2004 shortly before having season-ending elbow surgery. Watch Clemens say Pettitte "misheard" a remark several years ago »
Pettitte initially withheld details of the second incident because he got the HGH from his father, who had obtained it to help overcome health problems, Pettitte's attorneys said. Pettitte "sought to shield him from publicity," read a statement.
Pettitte, who has played with Clemens for the Yankees or Houston Astros since 1999, was excused by the committee Tuesday, along with former Yankee Chuck Knoblauch. McNamee told the panel he injected both men with performance-enhancing substances. See the key players in the case »
"Mr. Knoblauch and Mr. Pettitte have answered all the committee's questions and their testimony at the hearing is not needed," read a statement from the panel. Waxman said Pettitte's and Knoblauch's testimony backed up McNamee's account. Read Sports Illustrated analysis of the latest developments
Clemens' attorneys have steadfastly denied their client used performance-enhancing substances.
Clemens was careful in his remarks to attack McNamee's credibility, rather than the Mitchell report's integrity.
"If I am guilty of anything, it is of being too trusting of everyone, wanting to see the best in everyone and being too nice to everyone," Clemens said.
Clemens' legal team has filed a lawsuit against McNamee, saying his allegations "fueled rampant speculation and irreparably tainted the reputation of one of baseball's hardest working and most talented pitchers."
McNamee's attorneys last week, however, showed reporters photos of needles and gauze that McNamee said were used to inject Clemens with steroids and human growth hormone. They said they turned the items over to federal investigators in January.
McNamee, a former New York City police officer, kept the items because he was nervous that Clemens' drug use would become public and Clemens would turn against him, said Richard Emery, who is representing the former trainer.
Rusty Hardin, Clemens' lawyer, dismissed the purported evidence as "the desperate Hail Mary of a man who wants to ruin Roger." Hardin would not indicate whether Clemens would voluntarily provide a DNA sample but said, "Any time any legitimate law enforcement agency asks a reasonable inquiry of us, we'll be glad to respond."
Waxman said Wednesday that McNamee has acknowledged he was not forthcoming with investigators in past statements.
Rep. Dan Burton questioned why the panel should believe McNamee when he has lied repeatedly to investigators and the media. McNamee acknowledged lying when he previously said he'd never had dealings with steroids and that Clemens and Pettitte had never asked for any.
"You're here under oath, and yet we have lie after lie after lie," Burton alleged. McNamee did not get a chance to respond.
McNamee previously told the panel he withheld information about Clemens because he was "trying not to hurt the guy," Waxman said. Watch McNamee tell the panel, "I did tell the truth" »
The chairman also said that Clemens' past remarks included "conflicts and inconsistencies" on whether he had discussions about human growth hormone with McNamee.
Clemens met individually Thursday with House lawmakers, including Waxman.
Wednesday's hearing comes a day after the House committee took testimony from four doctors who testified on "the myths and facts about performance-enhancing substances such as human growth hormone, B-12 and other substances." E-mail to a friend