A chemical in many office copying machines could prove a cancer hazard to those heavily exposed over long periods, University of Texas scientists have found.

All the evidence so far is based on laboratory tests, "and we need some animal data before we can say whether it's a problem or not," said the head of the Texas group, Dr. Marvin Legator.

Just the same, officials of Xerox Corp., one of the largest copier makers, said they acted to reduce the amount of the suspect material in "toning" -- darkening -- chemicals sold since early March.

Scientists in Legator's laboratory at the University of Texas Medical Branch, Galveston, call the chemical -- nitropyrene -- a potential carcinogen or cancer-causer on the basis of tests in bacteria and in cultures of mouse cells.

Xerox officials said they had found adverse effects in similar tests, but no human health hazard. They said the chemical was a contaminant in the "carbon black" toner in copies, found at 10 to 15 parts per million in some toners. Since its findings, Xerox said, the firm that supplies the chemical to it and other manufacturers has reduced the level to about 0.15 parts per million, "an amount so small it's not precisely measurable."

Legator said he sees no likely risk for someone using a copier occasionally.

If current animal tests confirm the laboratory results, he said, he would "worry" aboue people who have worked all the time in confined copying areas.

Though the current toner has been improved, he also said that "may not be the end of the story," because carbon black toners contain other chemicals that need study.

Legator and an Environmental Protection Agency spokesman here said Xerox should have told EPA about its test results, as required by law if a potential hazard is identified. Xerox reported its results to a scientific meeting last March.

"We're concerned that the company didn't come to us with this information as soon as they had it," said Larry O'Neill, an EPA spokesman. "We don't even have the complete studies yet, just the abstracts. We've asked Xerox to submit them. Then we'll consider further steps."