title: Bioassay of malaoxon for possible carcinogenicity.
abstract: A bioassay of malaoxon, the oxygen analogue of malathion (an organophosphate insecticide), for possible carcinogenicity was conducted by administering the test chemical in feed to F344 rats and B6C3F1 mice. Groups of 50 rats and 50 mice of each sex were fed diets containing 500 or 1,000-ppm malaoxon for 103 weeks and were then observed for up to an additional 2 weeks. Matched controls consisted of groups of 50 untreated rats and 50 untreated mice of each sex. All surviving animals were killed at 103 to 105 weeks. The only effects that could be related to administration of malaoxon at the doses used were increased mortality among male mice, decreased mean body weights of female mice, gastric ulcers in male and female rats, and possibly C-cell adenomas or carcinomas of the thyroid among treated female rats. The incidence of C-cell adenomas or carcinomas among historical controls, however, precluded relating the incidence of these tumors to administration of the chemical. It was concluded that under the conditions of this bioassay malaoxon was not carcinogenic for F344 rats or B6C3F1 mice.