Endosulfan is a synthetic chlorinated cyclodiene and was introduced in 1956 as a broad spectrum insecticide.
A bioassay of technical-grade endosulfan for possible carcinogenicity was conducted using Osborne-Mendel rats and B6C3F1 mice. Endosulfan was administered in the feed, at either of two concentrations, to groups of 50 male and 50 female animals of each species.
The time-weighted average high and low dietary concentrations of endosulfan were, respectively, 952 and 408 ppm for the male rats, and 445 and 223 ppm for the female rats. In mice the high and low time-weighted average concentrations were, respectively, 6.9 and 3.5 ppm for the males and 3.9 and 2.0 ppm for the females. Twenty animals of each sex and species were placed on test as controls. The bioassay of high dose male rats was terminated during week 82, and the bioassay of low dose male rats was terminated during week 74. After a 78-week period of chemical administration, observation of female rats continued for 33 additional weeks and observation of mice continued for 14 additional weeks.
At the doses administered to rats in this study endosulfan was toxic, inducing a high incidence of toxic nephropathy in both sexes and testicular atrophy in males.
In both species high early mortality was observed in the male groups and no conclusions concerning the carcinogenicity of endosulfan can be drawn from this part of the bioassay. However, survival among females of both species was sufficient for meaningful statistical evaluation of the incidence of late-developing tumors. It is concluded that under the conditions of this bioassay, technical-grade endosulfan was not carcinogenic in female Osborne-Mendel rats or in female B6C3F1 mice.