To study the toxic effect of alpha-amanitin from Amanita fungi on mice in vivo.The LD50 of alpha-amanitin was determined with intravenous and intraperitoneal injections of alpha-amanitin. The mice were then injected intravenously with LD50 dose of alpha-amanitin, and the viscera index was evaluated. The peripheral blood samples were collected 24 hours later to measure the hemogram and biochemical indicators. The histological changes of internal organs were examined. The alpha-amanitin in organ tissues were measured.The intraperitoneal and intravenous LD50 doses of alpha-amanitin were 0.742 mg/kg and 0.327 mg/kg bodyweight, respectively. The WBC, RBC and Hb decreased significantly, whereas the serum BUN and Crea increased significantly. The serum ALT, AST, TBIL and DBIL increased to 24.0, 9.6, 26.3 and 37.0 times of the levels of controls, respectively, 24 hour after the injection. The viscera indexes of liver and kidney also increased significantly, and focal necrosis was found in the tissue slices 48 hours after the injection. The poison compound alpha-amanitin was detected in liver and kidney tissue homogenates by RP-HPLC 48 hours after the injection.Serum BUN, Crea, ALT, AST, TBIL and DBIL are sensitive indicators for the toxicity of alpha-amanitin in vivo. The pathological changes of liver and kidney are very serious. The cyclopeptide alpha-amanitin could reside in the two organs for a long last toxic effect.