Malathion is an organophosphate insecticide considered to be suitable as a substitute for certain uses of DDT. U.S. consumption in 1974 was 16 million pounds, surpassing that of all other organophosphate insecticides except methyl parathion. Household applications accounted for approximately 10% of that volume.
A bioassay of malathion for possible carcinogenicity was conducted by administering the test chemical in feed to F344 rats.
Groups of 49 or 50 rats of each sex were fed diets containing 2,000 or 4,000 ppm malathion for 103 weeks and were then observed for an additional 2 or 3 weeks. Matched controls consisted of 50 untreated rats of each sex. All surviving rats were killed at 105 or 106 weeks.
No tumors occurred in the dosed groups of rats of either sex at incidences that could be related clearly to administration of the test chemical. Compound-related toxic effects were not observed in female rats at the doses used, but in males decreased mean body weights, increased mortality, gastritis, and gastric ulcers were dose related.
It was concluded that under the conditions of this bioassay, malathion was not carcinogenic in male or female rats, but the females may not have received a maximum tolerated dose.
Note: Malathion was previously tested in Osborne-Mendel rats and B6C3F1 mice administered in feed (See TR-24, reported 1979).