Although the effect of ultraviolet (UV) irradiation on immune response was reported to supress cellular immune response, the exact mechanism was not elucidated. As recent development in cytokine research progress, it is well-known that immune response is regulated by cytokines and especially cellular immune response is induced by interferon (IFN)-r and interleukin (IL)-12 which is mainly produced from lymphocytes and macrophages respectively. Therefore our purpose was to elucidate the UV effect on cellular immune response and its mechanism. We have investigated the changes of host resistance by injection of Listeria monocytogenes which is an intracellular parasite after UVB irradiation in C57BL/6 mice which is known to have relatively strong cellular immune response. In addition we also have investigated the changes in the production of IFN-r from lymphocytes and the production of tumor necrosis factor (TNF)-a and IL-12 from macrophages in mice by UVB irradiation. The increase of mouse spleen index and susceptibility of iisteria monocytogenes infection was correlated with the decreased production of IFN-r, TNF-a and IL-12, which was known to induce the suppression of cellular imrnune response. Korean J. Immunol. 21, 4: 353~360, 1999