Dioxathion, an organophosphorous insecticide, was selected for bioassay by the National Cancer Institute because of its widespread use on livestock and edible crops, and a lack of adequate chronic toxicity data.
A bioassay for possible carcinogenicity of technical-grade dioxathion was conducted using Osborne-Mendel and B6C3F1 mice. Dioxathion was administered in the feed, at either of two concentrations, to groups of 50 male and 50 female animals of each species. The high and low time-weighted average concentrations were, respectively, 180 and 90 ppm for male rats and 90 and 45 ppm for female rats. The high and low time-weighted average concentrations for male mice were 567 and 284 ppm, respectively, and for female mice were 935 and 467 ppm, respectively. After a 78-week period of chemical administration, observation of the rats continued for an additional 33 weeks and the mice were observed for an additional 12 to 13 weeks. For rats, 50 animals of each sex were placed on test as controls and fed only the basal diet, while for mice, 20 animals of each sex served as controls.
In both species adequate numbers of animals survived long enough to be at risk from late-appearing tumors.
A variety of neoplasms was observed in treated animals of both species; however, none of the neoplasms observed were either histopathologically unusual or in statistically significant incidences.
Under the conditions of this bioassay, dietary administration of dioxathion was not carcinogenic in Osborne-Mendel rats or B6C3F1 mice.