Michaelis-Menten and maximum velocity values have been obtained for the arvin-catalysed hydrolysis of fibrinogen, benzoyl arginine ethyl ester and toluene p-sulphonyl arginine methyl ester. Optimum conditions of temperature (50°) and pH (8.0) have been detemined for hydrolysis of the synthetic substrates. Enzymatic Hydrolysis of TAME is compet- ively inhibited by benzoyl arginamide. Toluene-o-sulphonyl fluoride and p-TSF, on the other hand, are non-competitive inhibitors. Rapid loss of enzymatic activity occurs after y-irradiation in argon and N20 saturated solutions. Thereare indications that e-aq. and OH radicals contribute to a similar extent towards inactivation. Gel Filtration of the irradiated enzyme reveals the production of several low molecular weight fagments. Extensive destruction of amino- acid residues occurs during irradiation in argon.