Eleven polysaccharides were isolated from a hot-water extract of fruiting bodies of Lyophyllum decastes Sing. by ion-exchange chromatography and gel permeation chromatography. Three polysaccharides (IV-I, IV-2, and IV-3) composed mainly of glucose showed marked antitumor activities against Sarcoma 180 and their average molecular weights were 305 kDa, 130 kDa and 14 kDa, respectively. From methylation analyses and C-13-NMR spectra, it was suggested that IV-I was a (1-->3)-beta-D-glucan-type polysaccharide, IV-3 was a (1-->6)-beta-D-glucan-type polysaccharide, and IV-2 was a (1-->3, 1-->6)-beta-D-glucan-type polysaccharide or a mixture of both polysaccharides. Increases in the number of peritoneal macrophages and third component of complement (C3)-positive fluorescent cells in mice treated with IV-1 suggested that the inhibitory effect on tumor growth is due to immunological host-mediated mechanisms.