Lentinus giganteus, one of edible and medicinal mushroom belongs to Pleurotaceae of Agaricales, has been known to contain some inhibitive substances on Sarcoma 180 and curative effect on high blood pressure. Neutral saline soluble (0.9% NaCl), hot water soluble and methanol soluble substances (hereinafter referred to Fr. NaCl, Fr. HW and Fr. MeOH, respectively) were extracted from fruiting body of the mushroom. In vitro cytotoxicity tests, crude polysaccharides were not cytotoxic against cancer cell lines such as Sarcoma 180 and HepG2 at the concentration of 10~2,000 µg/ml, but crude polysaccharides from Fr. NaCl was toxic to NIH3T3 at the concentration of 10~2,000 µg/ml. Intraperitoneal injection with crude polysaccharides showed life prolongation effect of 14.3~67.5% in mice previously inoculated with Sarcoma 180, respectively. Fr. NaCl exhibited the immuno-potentiating activity of B lymphocyte by increasing the alkaline phosphatase activity by 1.53~1.68 folds compared with control at the concentration of 50~200 µg/ml, respectively. The numbers of peritoneal exudate cells and circulating leukocytes were increased by 7.7 and 1.6 folds by injecting Fr. NaCl and Fr. MeOH into the mice at the concentration of 50 mg/ml body weight, respectively.