title: Bioassay of p,p'-ethyl-DDD for possible carcinogenicity.
abstract: A bioassay of p,p'-ethyl-DDD 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 were administered p,p'-ethyl-DDD at one of two doses, either 3,500 or 7,000 ppm, for 105 weeks. Matched controls consisted of 20 untreated rats of each sex. All surviving rats were killed at the end of administration of the test chemical. Groups of 50 male mice were administered p,p'-ethyl-DDD at one of two doses, either 2,500 or 5,000 ppm, for 105 weeks. Groups of 50 female mice were administered the test chemical at one of two doses, initially either 5,000 or 10,000 ppm. Because of excessive lowered body weights in the dosed groups of females, the doses for the females were reduced after 48 weeks to 1,000 and 3,000 ppm, respectively, and administration at the lowered doses was continued for 57 weeks. The time-weighted average doses for the female mice were 2,828 and 6,200 ppm. Matched controls consisted of 20 untreated mice of each sex. All surviving mice were killed at the end of administration of the test chemical. No tumors occurred in the male or female rats or in the male mice at incidences that could clearly be related to administration of the test chemical. In female mice, hepatocellular carcinomas or adenomas occurred at incidences that were dose related (P=0.011), but in direct comparisons the incidences in the individual dosed groups were not significantly higher than that in the corresponding control group. Although the occurrence of hepatocellular carcinomas or adenomas in the dosed female mice are not clearly related to the administration of the test chemical, the increased incidence of these tumors in the high-dose group suggests that the tumors may be related to the administration of p,p'-ethyl-DDD. It is concluded that under the conditions of this bioassay, p,p'-ethyl-DDD was not carcinogenic for male or female F344 rats or male B6C3F1 mice. However, the occurrence of hepatocellular carcinomas and adenomas in female mice was suggestive of a carcinogenic effect.