Benz(a)anthracene (BA) and its five possible trans-dihydrodiols were evaluated for determination of their skin tumor-initiating activity and their mutagenic activity in Chinese hamster V79 cells. In addition, the skin tumor-initiating abilities of five diol-epoxides of BA were tested. Results showed (+/-)-trans-3,4-dihydroxy-3,4-dihydrobenz(a)anthracene (BA 3,4-dihydrodiol) to be approximately 10 times more mutagenic than was BA and about 20 times more mutagenic than were the other possible dihydrodiols in the V79 cells cocultivated with irradiated hamster embryo cells. As a skin tumor initiator, BA 3,4-dihydrodiol was approximately 5 times more active than BA, whereas the other BA dihydrodiols were all less active tumor initiators. (+/-)-trans-3alpha,4beta-Dihydroxy-1alpha,2alpha-epoxy-1,2,3,4-tetrahydrobenz(a)anthracene was found to be approximately 20% more active as a tumor initiator than was BA 3,4-dihydrodiol, whereas the other diol-epoxides of BA were less active than BA itself. The results suggest that the bay-region diol-epoxide of BA may be the ultimate carcinogen and mutagenic form of BA.