A single oral dose of the lyophilized deathcap fungus Amanita phalloides (85 mg/kg body wt) caused gastrointestinal signs of diarrhea, retching, and vomiting in beagles after a latent period of 16 hr. The pathologic lesions; the increases in serum transaminase (GOT, GPT), alkaline phosphatase, and bilirubin, as well as the fall in prothrombin time all indicated that liver damage was maximal at about 48 hr after poisoning. Four of twelve dogs given A. phalloides died with signs of hepatic coma within 35 to 54 hr with the biochemical values in the survivors reverting to normal by the ninth day. Silibinin administration (50 mg/kg) 5 and 24 hr after intoxication suppressed the serum changes and the fall in prothrombin time. The degree of hemorrhagic necrosis in the liver was markedly reduced, and none of the silibinin-treated dogs died.