Linda Tripp is ready to answer grand jury questions about her taped conversations with Monica Lewinsky that touched off a sex and perjury investigation of President Clinton.

A cooperating witness for Whitewater prosecutor Kenneth Starr, Tripp triggered Starr's inquiry into the Lewinsky matter with her 20 hours of recordings. Tripp's appearance, scheduled for Tuesday, comes as Starr's office negotiates with Lewinsky's lawyers on a possible deal that would give her immunity from prosecution in return for her testimony.The conjunction of the negotiations and Tripp's testimony - certain to be unfavorable to Lewinsky - does not necessarily signal an attempt by Starr to pressure the former White House intern to cooperate.

"What does Tripp offer that we don't know about?" said Jane Sherburne, a former White House lawyer in the Clinton administration. "She taped Monica, she spent untold hours with prosecutors already. She's been cooperating with them. I see it more as tying up loose ends."

Starr spokesman Charles Bakaly declined to comment on Tripp's appearance, while Joe Murtha, one of Tripp's lawyers, confirmed that the grand jury subpoenaed Tripp on Thursday.

Friday, two floors above the grand jury, a U.S. Court of Appeals panel was hearing oral arguments on whether three Secret Service employees should be compelled to testify in the Lewinsky investigation.

Starr, who is trying to force their testimony, has won in a lower court but the Justice Department is appealing. The department contends a president could be assassinated if he mistrusted his bodyguards and asked them to keep their distance.

The grand jurors likely have heard some or all of the audio tapes that Tripp made of her conversations with Lewinsky. In those recordings, Lewinsky reportedly admitted a sexual relationship with the president, although the former intern and Clinton both have denied an affair in the Paula Jones sexual harassment lawsuit against the president.

Tripp may have more to tell the grand jury than just repeating what is on the tapes - which constitute only a fraction of what Tripp says were "hundreds of hours" of conversations over 15 months.