The lawyer, Earl Ward, said Pettitte talked about HGH with trainer Brian McNamee following a conversation with Clemens, who has denied that he used HGH or steroids. McNamee worked with both Clemens and Pettitte.
"We're hopeful based on Andy's reputation that he will corroborate Brian's statements with regard to Roger," Ward said in a telephone interview.
Pettitte's meeting with a congressional committee investigating drug use in baseball was postponed until Monday. He originally was slated to appear for a deposition or transcribed interview Wednesday but the date was changed Tuesday by the House Oversight and Government Reform Committee.
McNamee said in last month's Mitchell Report that he injected Clemens at least 16 times with steroids or HGH in 1998, 2000 and 2001. He said he injected Pettitte two to four times with HGH.
Pettitte admitted two days after the Mitchell Report was released that he tried HGH for two days in 2002 - before it was banned by players and owners.
Ward said the discussion he was referring to occurred at Clemens' house.
"Based on what we know, there was a situation where Andy was speaking to Roger in Brian's presence, then Andy came over to Brian and essentially said, 'Why didn't you tell me about this stuff?' He referred to HGH," Ward said. "Brian discouraged him and then several months later, when he (Pettitte) got injured, he came back and asked Brian about it, and that's when Brian injected him. We believe that based on the fact that Andy came to Brian and asked him about HGH, it was Roger who told Andy about HGH and that's why he asked Brian about it."
Richard Emery, another lawyer for McNamee, said his client and Pettitte also discussed steroids use by Clemens.
"Pettitte is certainly going to tell the truth and if he tells the truth everything will be fine," Emery said.
"There are a number of conversations where Pettitte and Brian talked about Clemens' use. I think there is everything to believe Pettitte is not a liar."
Jay Reisinger, Pettitte's lawyer, would not discuss what Pettitte would say.
"He hasn't testified yet, and I'm not going to comment on what he's going to testify about," Reisinger said.
Ward's claims about the discussion were first reported by The New York Times on its Web site.
The delay of Pettitte's deposition or transcribed interview was the latest switch in the schedule of meetings between witnesses and staff before the Feb. 13 hearing.
"Just a mutually agreeable postponement," said Keith Ausbrook, Republican general counsel for the committee. "It give us a little more time to prepare and gives him a little more time to prepare."
Also asked to appear at next month's hearing are Clemens, McNamee, former Yankees second baseman Chuck Knoblauch, and former New York Mets clubhouse employee Kirk Radomski.
"Mr. Pettitte is cooperating voluntarily with the committee, and we look forward to his testimony on Monday," panel chairman Henry Waxman and ranking Republican Tom Davis said in a joint statement. "We appreciate Mr. Pettitte's willingness to assist the committee."
Knoblauch now is scheduled for Friday and would be the first of the five Feb. 13 witnesses to provide a deposition or transcribed interview. He agreed to appear after a subpoena was issued.
Clemens is to follow Feb. 5, with McNamee down for Feb. 7, and Radomski on Feb. 12 - pending further changes to a repeatedly shuffled schedule.
Letters sent by Waxman and Davis to Clemens, Pettitte and Knoblauch on Jan. 16, requesting their appearances both at the hearing and a pre-hearing meeting, said: "The committee asks that you provide testimony about allegations in Senator George Mitchell's report ... that you and other Major League Baseball players used performance enhancing drugs during your professional baseball career."
Clemens, Pettitte and Knoblauch were among more than 80 players named in the Mitchell Report.
McNamee told Mitchell he acquired HGH from Radomski for Knoblauch in 2001, and that he injected the player with it. Knoblauch's major league career ended in 2002.
Radomski pleaded guilty in April to federal felony charges of distributing steroids and laundering money, and is scheduled to be sentenced Feb. 8.
The 35-year-old Pettitte has a 201-113 major league record and won four World Series championships with the Yankees. He also helped his hometown Houston Astros reach their first World Series.
Pettitte returned to the Yankees last season and went 15-9. This offseason, he put off retirement and agreed to a $16 million, one-year contract to play for the Yankees next season.