Washington ? Mark McGwire will join other past and present players today before a congressional committee investigating steroids in baseball, a spectacle the sport had hoped to avoid just weeks before opening day.

What the subpoenaed major leaguers might reveal under questioning by members of the House Government Reform Committee won’t be clear until the hearing. Jose Canseco’s request for immunity from prosecution was denied Wednesday, raising the possibility of players’ invoking their Fifth Amendment right to refuse to answer.

“No witnesses have been or will be granted immunity,” David Marin, a spokesman for committee chairman Tom Davis, said in an e-mail to the Associated Press.

But the panel’s ranking Democrat, Henry Waxman of California, said: “Not everything’s been fully resolved.”

The flurry of activity on the eve of the highly anticipated hearing also included the committee’s release of baseball’s new drug-testing agreement; sharp critiques of that plan from members of Congress; an agreement that two-time AL MVP Frank Thomas can testify via video conference call; and the setting of a Sept. 6 trial date in the Bay Area Laboratory Co-Operative steroid-distribution case in San Francisco.

The testing agreement, which the commissioner’s office turned over to the committee Monday, along with other subpoenaed documents, contains a provision that testing would be “suspended immediately” if the government conducted an independent investigation into drug use in baseball.

The still-unsigned, 27-page document also retains a provision that allows the commissioner to substitute fines for suspensions, including $10,000 instead of a 10-day ban for a first offense.