Lentinan, an antitumor polysaccharide from Lentinus edodes, was degraded to seven fractions by treatment with formic acid. The low molecular-weight fractions (I and II) showed no antitumor activity against sarcoma-180 solid-type tumor and the absorption maximum of Congo Red did not shift in their presence in 0.1M sodium hydroxide. The medium molecular-weight fraction III, which required the increase of doses (5 or 10 mg/kg) for inhibition of tumor growth, caused a little shift. On the other hand, the absorption maximum of Congo Red shifted largely by the presence of high molecular-weight fractions (IV approximately VII) which showed the inhibition ratio of over 95% in a dose of 1 mg/kg. Participation of molecular weight in the antitumor activity of polysaccharides which contain (1 leads to 3)-beta-D-glucan main chain was discussed.