We have investigated the development of skin cancer from exposure to ultraviolet (UV) radiation in C3H-nu/nu nude mice. Nude mice, nude mice reconstituted with thymuses, and nude mouse skin grafted onto normal haired mice had similar tumor incidences and rates of tumor development. All tumors were squamous cell carcinomas and both well-differentiated and poorly differentiated lesions occurred in each of the groups. Transplants of the tumors that developed in nude skin grew preferentially in immunosuppressed mice as compared with normal mice, indicating that tumors from each treatment group were antigenic. These results indicate that the presence or absence of a functioning thymus does not seem to influence UV carcinogenesis.