To find antitumor components from higher fungi, the cultured mycelia of Paxillus atrotomentosus were extracted with hot water. The water soluble fraction was purified and separated by DEAE-cellulose ion exchange chromatography and Sepharose CL-4B gel filtration method. The separated fractions(Fr.) were designated CR A, B, C and D. Fr. A showed the highest inhibition ratio of 68.51% among the five tractions at a dose of 20 mg/kg/day. When Fr. A was examined for immunopotentiation activity, it increased the amount of the superoxide anion from activated macrophages to 1.1 fold and the number of plaques in hemolytic plaque assay to 2.3 fold, respectively. Otherwise, it did not show direct cytotoxity in sarcoma 180. Delayed type hypersensitiyity reaction showed that the decreased footpad swelling of tumor-hearing was restored to the normal. These results indicate that antitumor activity was exerted through immunopotentiation. Its chemical analysis showed 86.36% polysaccharide, 1.52% protein and 1.64% hexosamine. The polysaccharide consisted of fucose, galactose, glucose, mannose and xylose. This component was named paxillan.