title: Bioassay of dibenzo-p-dioxin for possible carcinogenicity.
abstract: A bioassay of dibenzo-p-dioxin (UDD) for possible carcinogenicity was conducted by administering the test chemical in feed to Osborne-Mendel rats and B6C3F1 mice. Groups of 35 rats of each sex were administered UDD at one of two doses, either 5,000 or 10,000 ppm, for 110 weeks. Groups of 50 mice of each sex were administered the same doses for 87 or 90 weeks. Controls consisted of groups of 35 untreated rats of each sex and 50 untreated mice of each sex. All surviving male rats were killed at 110 weeks, all surviving male mice at 92 to 97 weeks, and all surviving female mice at 91 to 93 weeks. Mean body weights of the dosed male and female rats and mice were lower than those of the corresponding controls; the depression in the amount of weight gained in the dosed male mice was, however, relatively slight. Except for the male rats, survival at the end of the bioassay was lower in the dosed groups of both rats or mice than in the corresponding control groups. At week 90, at least 57% of the rats and 54% of the mice were still alive. Because the mean body weights and survival rates of the dosed animals were lower than those of corresponding controls and because there was an increase in the incidence of hepatotoxic lesions, the 10,000-ppm concentration administered to the rats and mice is considered to be the maximum tolerated dose. No tumors were induced in rats or mice of either sex at incidences that were significantly higher in the dosed groups than in the corresponding control groups. It is concluded that under the conditions of this bioassay, UDD was not carcinogenic for Osborne-Mendel rats or B6C3F1 mice.