Purified 1,8-dihydroxy-3-methyl-9-anthrone or chrysarobin was found to be an effective skin tumor-promoter in SENCAR mice, although, at the dose used, it was approximately 43-fold less active than 12-O-tetradecanoylphorbol-13-acetate (TPA). When non-promoting doses of chrysarobin were applied 30 min prior to each application of TPA, a marked potentiation in the promoting response to TPA was observed. Chrysarobin will provide a valuable tool for studying the mechanism of action of anthracene-derived mouse skin tumor promoters.