title: Bioassay of coumaphos for possible carcinogenicity.
abstract: A bioassay of coumaphos for possible carcinogenicity was conducted by administering the test chemical in feed to F344 rats and B6C3F1 mice. Groups of 50 rats of each sex and 50 mice of each sex were administered the coumaphos in the diet at one of two doses, either 10 or 20 ppm, for 103 weeks and then observed for 0-1 additional weeks. Matched controls consisted of groups of 25 untreated animals of each species and sex. All surviving animals were killed at 103-105 weeks. Mean body weights of the dosed female rats were lower than those of corresponding controls, while mean body weights of dosed male rats and of dosed male and female mice were essentially unaffected. No clinical signs that are typical of organophosphorus poisoning were reported in either rats or mice. Survival of the rats and mice was not affected by administration of the test chemical. The test animals may have been able to tolerate higher doses. Sufficient numbers of animals in all groups of the rats and mice were at risk for the development of late-appearing tumors. In both rats and mice, no tumors occurred in the dosed groups of either sex at incidences that were significantly higher than those in corresponding control groups. It is concluded that under the conditions of this bioassay, coumaphos was not carcinogenic for either F344 rats or B6C3F1 mice.