Isopropanol is shown to affect considerably the thrombin activity. Its low concentrations (5-15%) activate the hydrolysis reaction of benzoyl-1-arginine ethyl ester (BAEE) by thrombin, whereas incomplete uncompetitive inhibition of the enzyme is observed in the presence of 20% isopropanol. Alcohol in the concentration of 25% results in complete reversible inhibition of the clothing and esterase activity of the enzyme. Isopropanol in the concentration of 25% is able to suppress the thrombin autolysis and therefore it may be used as a reagent which stabilizes thrombin during its storage in the aquatic-salt solutions.