LOS ANGELES (AP) _ A world-renowned statistician admitted on the witness stand today that he made an error in deciding how to calculate some genetic matches of bloodstains in the O.J. Simpson case.

Bruce Weir, professor of genetics and statistics at North Carolina Sate University, said that in performing calculations on some DNA test results, he failed to include a genetic marker that may have actually been present.

The mistake benefited the prosecution by suggesting a greater possibility the stains contained Simpson's blood.

If the genetic marker is added to the calculations, it increases the number of people other than Simpson who could have contributed to certain stains involving more than one person's blood.

``If I did not include them (the markers), I am sincerely sorry. I am also embarrassed,'' Weir said under cross-examination by defense attorney Peter Neufeld.

The mistakes apply to four stains: three in Simpson's Bronco and one on a bloody glove found by police. The defense didn't suggest Weir made such an error in computations on other mixed stains or on stains from single sources, including blood drops leading from the bodies.

Deputy District Attorney Lisa Kahn said outside court Weir would re-calculate the numbers, and the jury would get the new calculations Monday. She did not think the prosecution was harmed.

``By taking a prestigious and renowned scientist like Dr. Weir who can admit making a mistake on a calculation, that will do nothing but enhance his credibility,'' Kahn said.

The defense attorney hit hard at the error.

``By failing to include the additional pairings in these samples and these items, which do not exclude Mr. Simpson, the numbers that are arrived at by you and put on that board are biased against Mr. Simpson, isn't that correct?'' Neufeld asked.

``As it turns out, it looks that way, yes,'' Weir replied.

Neufeld suggested Weir should have done calculations for the possibility that the marker was there, and then again for the possibility that it wasn't there.

After the jury was excused for the weekend, defense attorney Johnnie Cochran Jr. denied an allegation made by prosecutor Christopher Darden that Simpson had stopped taking an arthritis medication so his hands would swell before having to try on gloves.

``Mr. Simpson on every occasion has taken his medication, and the medical records at the jail verified that fact,'' Cochran told the judge.

Transcripts of a Wednesday sidebar quoted Darden as saying prosecutors were told that Simpson `` has not taken the stuff for a day, and it caused swelling in the joints and inflammation in his hands.''

Darden did not specify whether he was referring to the demonstration in which Simpson tried on the actual evidence gloves, which he struggled to pull on, or a new pair of the same type of gloves.

In a sidebar today, a source said, the judge fined Neufeld $250, apparently for defying his order that Neufeld stop mentioning a letter from renowned scientists that was previously ruled inadmissable.

In another development, the defense pressed its effort to question district attorney employees who conducted a practice cross-examination of Detective Mark Fuhrman to prepare the embattled homicide investigator for his courtroom grilling by F. Lee Bailey.

The defense said in court papers ``there is nothing improper'' in preparing a witness for cross-examination but claimed the right to tell the jury about it ``to later impeach the witness.'' Prosecutors are resisting defense efforts to subpoena Deputy District Attorneys Alan Yochelson and Terry White and law clerk Melissa Decker.

``The only possible conclusion is that the prosecutors in this case are consciously engaged in the suppression of evidence favorable to the accused,'' the papers say.

The papers were filed as Weir's testimony ground on.

Earlier, the prickly statistician bristled at a suggestion from the defense that his complicated calculations on genetic matches would be meaningless if laboratories messed up.

``Everything I have done, I believe, is meaningful, given the starting point,'' Weir said.

But Weir acknowledged his numbers _ which the prosecution contends strongly suggest various bloodstains came from Simpson and the murder victims _ do rely upon the results of laboratory work, and that all he did was perform the math based on those results.

``Anything that goes prior to (the math) is beyond my expertise,'' Weir said.

Earlier, Weir repeatedly took issue with the nature of Neufeld's questions, suggesting the queries were repetitious or irrelevant. Asked at one point if he ever ``asserted facts that turned out not to be correct,'' he answered: ``Well, you'll probably refresh me with some.''

But the professor didn't show as much overt hostility toward the defense today as he did during a hearing Thursday outside the jury's presence.

Weir was called to testify on the numbers behind the DNA results of blood stains that appear to contain the blood from more than one person. These mixtures were found on such Simpson case evidence as the glove found at Simpson's estate and the bottom of victim Ronald Goldman's shoe.

Weir said there was a range of 1 in 300 million to 1 in 1 trillion that two randomly selected people could have had the same blood mixture found on the shoe. On one portion of the glove, the frequency range was 1 in 6 million to 1 in 600 billion, he said. The numbers fluctuated depending on the mix and the kind of DNA testing used.

Weir's testimony, however, was overshadowed by the revelations contained in a transcript of Wednesday's sidebar conferences that were so mean-spirited the judge fined two attorneys $100 each.

In one sidebar, Darden offered a threat.

``At some point,'' he told Cochran Jr. during a sidebar discussion, ``we are going to have a picture of him with the gloves on.''

The gloves, of course, are the brown, size extra-large Aris Leather Lights prosecutors claim Simpson wore while murdering Goldman and ex-wife Nicole Brown Simpson.

Whether prosecutors actually have a picture of Simpson wearing the same gloves was unknown. But it is clear they are actively searching for one.

They have subpoenaed Simpson's old employer, NBC Sports, for video footage of Simpson at football games on cold days, NBC said Wednesday. Authorities in New York recently mailed prosecutors photos of a gloved Simpson at a Buffalo Bills game about five months before the June 12, 1994, murders, a law-enforcement source told The Associated Press.

Photographs of Simpson wearing leather gloves were published early in the murder case. One of them shows Ms. Simpson looking adoringly at Simpson, who is holding an NBC microphone during broadcast commentary of a Miami Dolphins-Dallas Cowboys game on Thanksgiving 1993. The leather gloves Simpson wore appear to be black; the cashmere-lined gloves in evidence are dark brown.

The picture quest reflects the prosecution's fixation on gloves since last week's courtroom demonstration in which Simpson struggled to put on the gloves found near the bodies and behind his mansion. Prosecutors suggested the pair shrank from being soaked in blood.