Chemical characterization of the extracts, and purification and structural characterization of an antitumor glucan obtained from liquid-cultured mycelium of Grifola frondosa are described in this paper. The mycelium (13 g dry weight/1 culture) was extracted successively with hot water (LMHW, 14.9 g/100 g mycelium), cold alkali (LMCA, 6.3 g/100 g mycelium), and hot alkali (LMHA, 4.5 g/100 g mycelium). Each extract was dialyzed and then the polysaccharide fraction was precipitated with ethanol. Another portion of the mycelium was incubated with a buffer composed of glucose (5.0%) and citric acid, pH 4.5, for 3 d and the supernatant was obtained (LELFD, 50 g/100 g mycelium). The brothe was dialyzed and the nondialyzable fraction was precipitated with ethanol (LLFD, 1.3 g/1 culture). All of the above fractions were composed of glucose as the major carbohydrate. Except for LMHW, all fractions showed antitumor activity against sarcoma 180 cells in ICR mice. The activity was the strongest in the LELFD fraction. Antitumor glucan (grifolan LE) was purified from LELFD by diethylaminoethyl-Sephadex A-25 chromatography and ethanol precipitation. Grifolan LE gave M_r of more than 5×10⁶ (8 M urea/0.2 N NaOH), [α]_D -6°--9° (c=0.1%; 0.3 N NaOH), and showed metachromasy coupled with Congo red. By carbon-13 nuclear magnetic resonance spectra and methylationanalyses, grifolan LE was found to be a branched β-1, 3-glucan containing a branch at C-6 of every three main chain glucosyl units.