The cancer-causing flame retardant, Tris, can be absorbed through the skin of thousands of children still wearing old, treated pajamas, even though use of Tris is now banned, scientists reported last week.

The finding contradicts a 3-to-2 determiniation in 1977 by the Consumer Product Safety Commission that repeated washings in effect removed so much Tris that treated sleepwear could generally be safely worn.

On the basis of the new studies, one of the main investigators, Dr. Evan Horning of Baylor College of Medicine in Houston, said in an interview, "I simply would not keep old pajamas that might have been treated. As a parent, I'd throw them away."

The five-member federal commission forbade new use of Tris on children's sleepwear in April 1977, but at the same time voted against making manufacturers spend of dollars to repurchase laundered sleepwear from consumers.

The commission majority relied on tests indicating that after three washings, 95 percent of Tris was removed from two polyester fabric samples. This was disputed by the Environmental Defense Fund, the group that first raised an alarm about Tris in 1976. It said other tests showed that 10 percent of the Tris was left on the fabric surface even after 10 washings.

The new findings were reported in part by Dr. Ralph Dougherty of Florida State University at an American Chemical Society meeting in Indianapolis. He summed up painstaking tests on 11 children by Drs. Bruce Ames and Arlene Blum of the University of California at Berkeley and by Horning, and chemical analyses by Dougherty and Horning.

Another report by Ames and others is scheduled to appear in Science magazine. Ames is the respected creator of the widely used "Ames test" for mutagenicity, or potential genetic damage by suspect chemicals.

"Our results show what we expected," he said, "but it was something that had to be demonstrated" since the 1977 ban was based solely on tests in which Tris caused kidney cancers in animals. Some clothing makers and retailers have been maintaining in court that there is no human proof that Tris can cause harm.

Blum and Ames first found the Tris metabolite or byproduct, dibromopropanol, in the urine of a California girl after she wore new Tris-treated pajamas just before the ban. Horning found the metabolite in the urine of 10 more children, some of whom had been wearing garments washed many times.

Horning and Doughterty used a new, difficult chemical method - negative chemical ionization mass spectrometry - to find the chemical in mere parts per billion.

Parts per billion is "the level at which there can be long-term toxicity with chronic exposure," Dougherty said.

"It's not just a matter of the chemical getting into the urine." Horning added. Most of the chemical is excreted from the body fairly soon, he said but: "We think some stays in the body fat, though we still must work on showing this.

"This whole problem still needs to be studied extensively, since some children may begin to develop cancers after 15 to 20 years."

Various scientists estimated last year that between 540 and 51,000 affected children might eventually develop a cancer, with the 51,000 estimate coming from Ames. He also warned of possible genetic damage.

Francine Shacter, manager of the Consumer Product Safety Commission's chronic hazard program, said there is no longer any Tris-treated sleepwear sold, and clothing marketed since April 1977 contains no Tris. She said products labeled as pure cotton or "modocrylic" were not treated with Tris, but those labeled as containing any polyesters, acetates or triacetates were more often treated than not.

"We have no adverse reports," she added, on any chemicals being used today to make garments fire retardant.