water-insoluble proteases were prepared by immobilizing papain and chymotrypsin onto the surface of the copoly(ethylene/acrylic acid) fiber. the effect of the surface area of the copolymer fiber on the amount of the immobilized protease was investigated. the mode of the immobilization between protease and copolymer such as a covalent fixation or an ionic interaction on the enzymatic activity and the stability of the immobilized protease was also investigated. the stability of the immobilized protease in this study meant thermal stability, durability for repeated use, stability in urea, and durability for repeated washing. the activity of covalently immobilized proteases was found to be still higher toward small ester substrates, but rather low toward casein, a high molecular weight substrate. the covalently immobilized proteases onto the copolymer fiber gave an almost constant specific activity, suggesting less structural deformation of the protease molecule than the conventional immobilized enzyme system in the lower surface concentration. the thermal stability of the immobilized proteases was higher than that of the respective ionic interaction or native proteases. the initial enzymatic activity of the covalently immobilized proteases was maintained almost unchanged without any elimination and inactivation of proteases when the batch enzyme reaction was performed repeatedly, indicating excellent durability. (author abstract) 22 refs