A new anti-tumor polysaccharide, lentinan (beta-1, 3 Glucan) was studied on the acute toxicities using both sexes of mice (ICR) and rats (CD) treated by intravenously (i.v.) intraperitoneally (i.p.), subcutaneously (s.c.) and orally (p.o). LD50 values in mg/kg body weight were essentially the same regardless of species as well as sexes and estimated as follows: 250-500 (i.v.) greater than 2500 (i.p., s.c., and p.o.). Cyanosis, convulsion and death were observed in both species of animals administered (i.v.) with only higher dosages of lentinan. No remarkable toxic signs being specific from lentinan were observed in any cases of treatment, i.p., s.c., and p.o. Gross findings: enlargement of the spleen (i.v., i.p., s.c.) and coarse nodular surface of the kidney (i.v.) in the both species of animals, erythema of the ears (i.v., i.p., s.c.) in mice, mesenteric petechial hemorrhage of the lung and abdomen (i.v.) enlargement of the mesenteric lymph nodes (i.v.) and edema of the diaphragm and intestine (i.p.) in rats were observed. In parallel, another sample of lentinan for clinical use prepared by freeze-dried procedure was tested in both sexes of mice and rats treated by i.v. alone, comparing with a original sample mentioned above. So far as the acute toxicities of lentinans concerned, no significant differences between two preparations were observed.