Fate of lentinan, which is one of antitumor polysaccharides, was investigated with 3H-lentinan. 3H-Ientinan was prepared by Wilzbach 3H gas exposure labeling, then purified by the gel filtration chromatography. 1) After a single intravenous injection of 3H-lentinan, radioactivity in plasma declined in a biphasic process. 2) There were urinary and fecal excretion, but expiratory excretion was a small proportion. 3) The radioactivity was predominantly incoporated into the liver and spleen, indicating selective retention of the lentinan in reticuloendthelial system cells. On the other hand the level of radioactivity in the lung and kidney was dropped rapidly. 4) We can not find the selective incorporation of radioactivity into the tumor. 5) There was no species difference on the distribution of lentinan among mouse, rat and dog. 6) These results obtained were similar to other polysaccharides and immunologic antitumor agents.