The purpose of these studies was to determine whether ultraviolet radiation contributes to the induction of cutaneous melanocytic tumors in mice. At 4 days of age, C3H/HeNCr(MTV-) mice received an initiating dose of 7,12-dimethylbenz(a)anthracene, followed by twice weekly applications of croton oil. Of the mice treated with this protocol, 31% developed melanoma. The addition of chronic doses of ultraviolet radiation (UVR) to the carcinogen treated site dramatically accelerated the development of the melanomas. Substituting UVR for the initiator or the promoter in this two-stage carcinogenesis model resulted in the development of a few melanocytic tumors (less than 10%). These experiments demonstrate that UVR accelerated the development of cutaneous melanocytic tumors induced in mice by chemical carcinogens and participated in the induction of these tumors by serving as a weak initiator or promoter.