Evidence is presented to show that mice treated with various regimens of 8-methoxypsoralen followed by exposure to long-wave ultraviolet light (PUVA) are rendered tumor-susceptible when challenged with short-wave ultraviolet light (UVB) induced regressor tumors. These same tumors are readily rejected when implanted into normal syngeneic animals. Similar observations have been made in mice treated with subcarcinogenic doses of UVB, where it was shown that the tumor-susceptible state is mediated by suppressor T lymphocytes. These suppressor T-cells may be generated in response to antigens expressed by UVB damaged skin cells. It is now known that suppressor T-cells generated in mice treated with UVB are Ia-positive and have specificities for cross-reacting tumor antigens shared by all UVB-induced tumors, which have been tested to date. Our data suggest that, like UVB treated mice, treatment of mice with PUVA results in tumor-susceptibility mediated through the generation of Ia+ suppressor cells. Since PUVA treatments appear to generate a suppressor cell response in mice, a possible mechanism by which these treatments act to manage autoimmune type skin diseases, such as vitiligo, is discussed.