The authors isolated an endotoxin-like substance from the mycelium of Aspergillus fumigatus originating from a man and a penguin of aspergillosis by means of alcohol precipitation. The chemical nature of the fraction seems to be mainly protein. The toxicity is very low but lethal for mice in larger dosis and even in smaller dosis, a characteristic lesion, that is granuloma, is produced in various; organs; especially lung and liver, and spleen, kidney and intestine. Hemorrhagic change for skin, Shwartzman reaction and hemolysis are occurred. Antigenicity and anaphylactic shock are observed. Such toxic substance was also isolated from the culture filtrate of the strains used. The histological findings in mice due to both the two substances from the mycelium and culture filtrate are quite similar to those caused by the killed and living cells of the same strains. Thus this endotoxin-like substance may be considered to play an important role in Aspergillus infections.