LOS ANGELES (AP) _ It happened during the William Kennedy Smith trial. It was a big issue in the Joey Buttafuoco-Amy Fisher furor. And in the O.J. Simpson case, it may already have cost prosecutors a key witness.

In high-profile criminal cases around the country, prosecutors are increasingly being hamstrung by witnesses' desire to get paid to talk to TV shows.

''It just really kills a witness' credibility,'' said Fred Klein, who prosecuted Fisher. ''Getting paid to tell your story inevitably makes a juror wonder - and even makes me as a prosecutor wonder - is this story really true?''

Simpson is accused of murdering his ex-wife and a friend of hers. He has pleaded innocent.

Last week, Jill Shively told the syndicated TV show ''Hard Copy'' that she saw Simpson speeding through the streets of Brentwood near the crime scene around the time of the slayings.

Shively also testified before a grand jury, which at the time was considering a Simpson indictment.

Her lawyer, James M. Epstein, wouldn't say what she told jurors. But he said he doesn't believe Shively will be called to testify when Simpson goes to trial.

Shively's testimony may have been tainted because she or a representative contacted the TV show ''Inside Edition,'' offering to give an interview for $100,000, sources speaking on condition of anonymity told The Associated Press.

District Attorney Gil Garcetti refused to say whether Shively will testify at a preliminary hearing on Thursday.

But Garcetti must have been ''putting his head in his hands and crying,'' said John Shepard Wiley Jr., a former federal prosecutor and now a law professor at the University of California at Los Angeles.

''That person just torched themselves. They have very, very badly hurt their benefit to the prosecution,'' Wiley said.

''Hard Copy'' refused to comment on the case. ''All I can say is that 'Hard Copy' stands by its reporting,'' said spokesman Gary Rosen.

In fact, ''Hard Copy,'' ''Inside Edition'' and other syndicated TV news shows won't even confirm they pay for interviews.

But a golf caddie who, on a ''A Current Affair,'' described Simpson's behavior the day of the slayings refused to talk to other news organizations, saying he was ''under contract.'' Most, including the AP, refuse to pay for interviews.

In at least one case, payments for interviews are on the record.

Three years ago during the William Kennedy Smith rape trial in Florida, a friend of Smith's accuser testified she rushed to the Kennedy estate after her friend called for help, and found her crying hysterically.

On cross-examination, Smith's attorney attacked Anne Mercer sharply because she had accepted $40,000 to tell her story to ''A Current Affair.'' The figure drew gasps in the courtroom.

Smith's attorney, Roy Black, suggested Mercer tailored her story to intrigue TV producers after she ''realized you could cash in on the Kennedy name.''

Mercer denied that. But she acknowledged she and a boyfriend used the money for a six-day trip to Mexico.

Prosecutors obviously knew the woman was vulnerable on the money issue, Wiley said.

''Some witnesses are so important, you almost have to use them and take the chance,'' Wiley said. Smith was acquitted.

In the Amy Fisher case, the issue never came up at trial because Fisher eventually pleaded guilty to shooting Buttafuoco's wife.

But TV payments were ''a big, big issue'' as prosecutors first investigated whether they had enough evidence to try Fisher, Klein said.

''People were constantly going on television with information before we had even had an opportunity to talk to them,'' he said. ''It was very frustrating.''

If the case had gone to trial, the payments ''certainly might have influenced our decision whether to use them as witnesses.''

Can prosecutors do anything to stop TV payments?

It would not be witness-tampering ''to suggest that they not go on TV and be paid,'' Klein said. ''But you certainly can't require it.''

Some defense attorneys - never especially sympathetic to prosecutors' plight - insist the problem isn't really new.

For years, attorneys in civil cases have cast doubt on testimony by expert witnesses, trying to persuade jurors that the expert appeared just to pick up a paycheck.

And many witnesses who take the stand in criminal cases are of questionable integrity because they're testifying to avoid prosecution themselves.

''The fact that a witness has spoken on TV, for financial consideration, doesn't make that witness any less truthful than somebody who's made a deal with prosecutors to save themselves,'' said New York defense attorney Barry Slotnick, who represented subway gunman Bernhard Goetz.

But if he faced a witness paid to talk on TV, would he bring it up?

Slotnick's reply came quickly: ''Of course.''