Lentinan, a polysaccaride extracted from the edible mushroom Xiang Gu (Shitake), has antiviral, antitumor, anticoagulative, antioxidative and immunoenhancing effects in several species. The objective of this randomized and controlled experimental study was to determine the acute and subacute toxicity of lentinan in mice. Three groups of 10 mice were given a single dose of 240 mg, 120 mg or 60 mg lentinan respectively and no deaths were observed after 7 days. There were no deaths in 20 mice given 720 mg in one day, but some mice became depressed and had reduced activity, which resolved within 7 days. The maximal tolerated dose of lentinan was calculated as 36,000 mg/kg/day of body weight. If the LD50 is more than 5,000 mg/kg body weight, administered once by mouth in the rat, the drug is considered nontoxic. To determine the subchronic toxicity of lentinan, 80 mice were randomly divided into 4 groups and administered 240 mg, 120 mg or 60 mg of lentinan solution or saline respectively once daily for 14 days. On days 1 and 14 after lentinan was discontinued, complete blood counts and serum biochemical tests of liver and kidney function were evaluated on 6 mice from each group and necropsy including histological examination and organ index calculations were performed on 10 mice from each group. Minimal transient clinicopathological changes and no gross or histological changes were found in any major organs. The hypothesis of the study was accepted that lentinan would have minimal to no acute or subchronic toxicity.