Russula rosacea, one of edible and medicinal mushroom belonging Agaricales of Basidiomycota, has been known to have good inhibitory effects on sarcoma 180 and Ehrlich carcinoma of mice. 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, HepG2, HT-29 and NIH3T3 at the concentration of 10~2000 µg/ml. Intraperitoneal injection with crude polysaccharides exhibited life prolongation effect of 21.4~45% in mice previously inoculated with Sarcoma 180. Fr. HW improved the immuno-potentiation activity of B lymphocyte by increasing the alkaline phosphatase activity by 6.8 fold compared with control at the concentration of 500 µg/ml. In case of Fr. NaCl, the numbers of peritoneal exudate cells and circulating leukocytes were increased by 6 and 2.6 folds at the concentration of 50 mg/kg, respectively. Therefore, the antitumor effect against mice Sarcoma 180 by Russula rosacea could be due to immunomodulating activity.