title: Bioassay of N,N'-dicyclohexylthiourea for possible carcinogenicity.
abstract: A bioassay of N,N'-dicyclohexylthiourea for possible carcinogenicity was conducted by administering the test chemical in feed to Fischer 344 rats and B6C3F1 mice. Groups of 50 rats and 50 mice of each sex were administered N,N'-dicyclohexylthiourea at one of two doses, either 25,000 or 50,000 ppm, for 109 weeks for rats or 104 weeks for mice. Matched controls consisted of 50 untreated rats or 50 untreated mice of each sex. Mean body weights of male rats and male mice were unaffected by the compound, whereas mean body weights of the females of each species showed mild dose-related retardation over the bioassay period, when compared with the matched controls. Survival was sufficient to termination of the study in all groups of both rats and mice for the development of late-appearing tumors. In male rats there was an increased incidence of hyperplasia of the follicular cells of the thyroid (males: controls 3/43, low-dose 16/49, high-dose 15/49; females: controls 1/48, low-dose 7/48, high-dose 5/49). The incidences of tumors of the follicular cells of the thyroid, although increased among the dosed male rats, were not statistically significant in either sex. In mice, a variety of neoplasms of the type usually encountered in the B6C3F1 strain were observed in both dosed and control animals. None of the tumors occurred at statistically significant incidences. Follicular-cell hyperplasia of the thyroid was observed at an increased incidence in both the dosed males and females (males: controls 3/39, low-dose 12/46, high-dose 9/45; females: controls 8/38, low-dose 22/46, high-dose 21/46). An increase in proliferative lesions of the follicular cells of the thyroid was associated with the administration of N,N'-dicyclohexylthiourea in both Fischer 344 rats and B6C3F1 mice. However, because statistical significance was not achieved and because thyroid tumors are not rare, spontaneous lesions in these strains of animals and occur with a variable incidence, it is concluded that under the conditions of this bioassay, N,N'-dicyclohexylthiourea was not demonstrated to be carcinogenic in either species.