A polysaccharide was extracted from fruit bodies of Grifola frondosa (maitake), and the chemical structure and antitumor activity were studied. The extracted polysaccharide could be hydrolyzed by ?A-glucanase into glucose, indicating it to be a β-glucan. The sample gave methyl 2, 3, 4, 6- tetra-Ο-, methyl 2, 4, 646-Ο-, methyl 2, 3, 4-tri -Ο- and methyl 2, 4-di-Ο-methylglucoside in the molar ratio of 4 : 2 : 1 : 4 on methylation. In the carbon-13 nuclear magnetic resonance spectrum, the signals of C-6' (related to (1-6) bonding) and C-3' (related to (1-3) bonding) were observed in addition to those of free C-6 and C-3. These results indicate that the major chain is made up of β-1, 6-linked glucose residues with branches of β-1, 3-linked glucose. This glucan inhibited the growth of Sarcoma 180 tumor in ICR mice.