In this study, we investigated the immunopotent activities of lepidan, a water soluble proteoglycan from Lentinus lepideus. To examine whether lepidan may affect nonspecific immune responses, we determined the effect on the production of nitric oxide (NO). Lepidan effectively increased the NO production in IFN-${gamma}$ and LPS triggered RAW 264.7 cells. To further demonstrate the evidence that lepidan activates various immune cells, we determined the alkaline phosphatase activity, plaque-forming cell number and secretion of interleukine-4 (IL-4) and granulocyte/macrophage-colony stimulating factor (GM-CSF) after lepidan treatment in murine splenocytes. The results showed that lepidan increased alkaline phosphatase activity and the number of plaque-forming cells, which indicates that lepidan can lead B lymphocytes to late stage of differentiation. Also, when the splenocytes were cultured with lepidan for 48 hr, the level of IL-4 and GM-CSF in the supernatant dramatically increased. Taken together, these data suggest that lepidan is a biological response modulator that is able to activate immune responses.