A sugar substitute being promoted as a cavity fighter in Wrigley's new Orbit chewing gum has apparently caused cancers in some laboratory animals.

The report of this yesterday by an agency of the National Institutes of Health caused the State University of New York to halt a three-day-old project to test the gum as a cavity-reducer in 150 Long Island sixth-graders.

But David Sloane, assistant vice president of Wm. Wrigley Jr. Co. in Chicago, said I would keep selling its heavily advertised Orbit since it sees "no public health problem" on the basis of information so far.

The sweet but non-sugar chemical is xylitol, a chemical found in the body, where it metabolizes carbohydrates. It is also found in such plant products as birch bark, corn cobs, peanut shells, cottonseed and cottonseed oil.

A Finnish company makes it from birck bark, and it is imported by Hoffman-LaRoche Inc., A. Nutley, N.J., pharmaceutical firm.

That firm hired Huntington Research Center near London, England, to do animal tests. Last Wednesday Hoffman-La Roche told federal officials of "preliminary" data indicating that two years of feeding the chemical to rats, mice and dogs had produced urinary tract abnormalities, kidney stones and tumors - some cancerous - in a number of the animals, though none of these problems appeared in all the species.

Dr. James Carlos, associate director of the National Institute of Dental Research, flew of Long Island to say at a news conference that the institute advised the state university at Sony Brooks to discontinue the school childrens' study, at least temporarily. The institute funded what was planned as a three-year, $210,000 project to try it, confirm a 1975 Finnish study in which children who chewed xylitol gum had up to 85 per cent fewer dental cavities than a control group.

Carols said there was "clearly" no hazard yet to the sixth graders, since the most any chewed so far was nine sticks. He said FDA officials are flying to England to look more closely at the laboratory results before considering any formal action.

Several food firms have been at least thinking of putting the chemical in their products, it was learned.

Wrigley officials maintained that the animals developed tumors only when they ate as much xylitol as they'd get in 200,000 sticks of gum a year. But many similar cancer tests in animals are considered valid, since researchers would have to give smaller amounts of a chemical to thousands of animals to duplicate the usual sort of human consumption.