3H-labeled lentinan was prepared by methylation with 3H-dimethyl-sulfate. Blood level, distribution and excretion were studied in rats. After intravenous injection of [methyl-3H] -methyl lentinan, radioactivity rapidly disappeared from blood, lung and kidney, distributed mainly in liver, spleen and mesenteric lymph nodes. There was urinary excretion, but expiratory excretion was a small proportion. In the case of multi-injection of [methyl-3H] -methyl lentinan distribution of radioactivity was similar to a single injection, but retention of 3H was less than a single injection. Bile excretion and transport to fetuses and milk were in mere trace amounts. No substantial difference was found between two types of the 3H-labeled lentinan in either the distribution or excretion.