Studies over the past two decades have established that exposure to ultraviolet radiation (UVR) has profound effects on immunity. Most of this work was done in animal models with limited data from human studies. Exposure to UVR inhibits the induction of contact hypersensitivity in mice systemically at high doses and locally at low doses, associated with the appearance of transferable T suppressor cells. In some strains of mice, chronic UVR exposure results in the occurrence of highly immunogenic cutaneous malignancies which are rejected upon transplantation to syngeneic recipients. However, within the primary host the tumor is protected from destruction, at least in part, by the appearance of T suppressor cells. Syngeneic animals exposed to large doses of UVR also permit the growth of these transplanted tumors and this phenomenon also results, at least in part, from the presence of suppressor cells. Interestingly, these suppressor cells appear to recognize UVR-induced regressor tumors as a class within a given mouse strain, suggesting that they recognize shared determinants. The mechanisms of these examples of UVR-induced immunosuppression are not completely clear, but perhaps relate to changes in Langerhans cell function and/or epidermal cell release of cytokines induced by UVR. Exposure to UVR in vitro alters the ability of epidermal cells and Langerhans cells to present some antigens. Keratinocytes alter their secretion of specific cytokines after exposure to UVR and are induced to produce immunosuppressive factors. Immunosuppressed patients also have increased rates of skin cancer, suggesting immunologic involvement in regulation of cutaneous oncogenesis in humans.(ABSTRACT TRUNCATED AT 250 WORDS)