Skin tumors induced in mice by UV light are usually immunologically rejected by normal syngeneic recipents. We evaluated the immune status of primary hosts against these highly antigenic tumors immediately after surgical removal of the primary tumor. All primary hosts were susceptible to challenge with their autochthonous tumors, though most of these were rejected by untreated control mice. Primary hosts were also susceptible to challenge with isografts of antigenically dissimilar UV-induced neoplasms. The susceptibility of the primary hosts to tumor challenge was probably induced by chronic exposure to UV light, since UV-irradiated non-tumor-bearing mice were also susceptible to challenge with these tumors. Although UV-treated mice were unalbe to reject these syngeneic tumors, they could reject skin and tumor allografts. Further, UV irradiation did not interfere with the second-set rejection of syngeneic UV-induced tumors in mice that were specifically immunized before UV treatment.