Acute toxicity of L-lysine-alpha-oxidase, an antitumor enzyme of fungal origin was studied. After intravenous administration in a dose of 3000 U/kg the substance induced no signs of intoxication in the laboratory animals or their death. Direct dependence of the vessel permeability increase on the enzyme concentration was determined. In experiments on rabbits and guinea pigs L-lysine-alpha-oxidase showed no local irritating effect.