Silybin is known to be a major active flavonoid component isolated from Silybum marianum, a hepatoprotective medicinal plant. In this study, we examined the immunomodulatory role of silybin on T cell and macrophage-mediated immune responses. To do this, the proliferation of splenic lymphocytes and CD8+ CTLL-2 cells under mitogenic stimulation with lipopolysaccharide (LPS), concanavalin (Con) A and interleukin (IL)-2 and the production of $TNF-{alpha}$ and NO from LPS- and $IFN-{gamma}$-activated macrophages was evaluated under silybin treatment. The mitogenic proliferation of splenic lymphocytes induced by LPS and Con A was strongly diminished by silybin in a dose-dependent manner. Moreover, the proliferation of CD8+ CTLL-2 cells was also negatively modulated by the compound. In contrast, silybin did not strongly suppress the proliferation of normal splenocytes and T cell line Sup-T1 cells, indicating that the inhibitory effect of silybin may be due to blocking only mitogenic responses of splenic lymphocytes. In addition, silybin inhibited $TNF-{alpha}$ production in LPS-stimulated RAW264.7 cells. Effect of silybin however was distinct, according to NO-inducing stimuli. Thus, silybin only blocked NO production induced by $IFN-{gamma}$ but not LPS and the inhibition was increased when PMA was co-treated with $IFN-{gamma}$. Unlike NO inhibition, however, this compound protected the cytotoxic damage of RAW264.7 cells induced by both LPS and $IFN-{gamma}$. Therefore, our data suggest that silybin may participate in host immune responses mediated by T cells and macrophages via regulating mitogenic proliferation, and the production of $TNF-{alpha}$ and NO, depending on cellular stimuli.