Human urokinase was immobilized on an ethylene vinyl acetate copolymer surface. Soluble urokinase showed its maximum activity at pH 8.5, while the immobilized enzyme was most active at pH 9.0. Apparently, the shift in optimal pH was due to the polyanionic nature of the carrier surface on which the enzyme was immobilized. Optimal temperatures of soluble urokinase and immobilized enzyme were identical, i.e., 37 degrees C. The stability of immobilized enzyme against thermal degradation was several times higher than that of the soluble enzyme. Its stability at higher temperatures is one of the main reasons for the clinical use of immobilized urokinase as an antithrombotic material.