In this study lymphocytes from blood and/or spleen of different species (rat, mouse, human) were exposed to different doses of ultraviolet radiation (UVR). The functional activity of these lymphocytes was determined using assays for mitogen proliferation and the mixed lymphocyte response (MLR). These experiments demonstrated that in vitro exposure to UVR causes a dose-dependent decrease of the MLR activity of the irradiated lymphocytes. Viability of lymphocytes and mitogen proliferation responses were also decreased by UVR exposure but less severe in comparison to the MLR. Lymphocytes of rats seem to be more sensitive to UVR as compared to lymphocytes of mice and humans.