To find antitumor components in the cultured mycelia of Lepiota procera, the proteinpolysaccharide obtained from the mycelia was subjected to DEAE - Sephadex A-50 column chromatography and Sepharose-4B gel filtration. Of the fractions, the purified Fraction C1 was named lepiotan and examined for antitumor activity against the solid form of sarcoma 180 in ICR mice. The inhibition ratio of lepiotan was 64% at the dose of 10 mg/kg/day for 10days. The chemical analysis of lepiotan showed 60% polysaccharide and 21 % protein. The polysaccharide moiety was found to be a heteromannoglucan which consisted of 46.3% glucose, 40.2% mannose and 11.0% fucose. When the antitumor component, Fraction A, was examined for immunopotentiation activity, it was found to increase the number of plaques in hemolytic plaque assay and to restore the immunity in the tumor-bearing mice up to 89.7% of the normal level. Also the antitumor acitivity was suppressed by the treatment with carrageenan, an antimacrophage agent. These results indicate that the antitumor activity was exerted through immunopotentiation, but not through direct cytotoxicity against the tumor.