A protein-polysaccharide fraction of a Korean wild mushroom Psathyrella velutina, PVP, was prepared and its antitumor immunomodulatory activity was investigated. When PVP was administered once daily for seven days from day 1 to day 7 into male ICR mice implanted with 1 $ imes$ 10$^{5}$ cells of sarcom 180 tumor cells into the peritoneum on day 4, it inhibited the growth of sarcoma 180 cells by 92.8%. In XTT assay, PVP also exhorted in vitro anti-proliferation activity on U-937, a human monoblastoid cell line, as well as sarcoma 180 cells. PVP showed marked stimulatory activity on the immune system in that it induced the accumulation of PEC (the stimulation index, Sl=4.90 at 100 mg/kg), stimulated the BALB/c mouse splenic lymphocytes to form lymphoblasts (Sl=5.75 at 100$mu extrm{g}$/ml), and upergulated the expression of CD25 molecules. All these results strongly support that PVP exhorts its antitumor activity through stimulation of the immune system as well as anti-proliferative activity on the tumor cells.