The surface of polyacrylonitrile hollow fibers were hydrolyzed and covalently bonded with cholesterol oxidase (COD) via glutaraldehyde. The immobilized amount of the COD increased with the concentration of glutaraldehyde. However, COD immobilized with 10% glutaraldehyde had higher activity than with other concentrations. The stabilities of immobilized COD to pH and temperature were higher than those of native enzyme. The immobilized enzyme retained 80% of initial activity after 15 days when stored at 4degreesC, which was longer than native COD. After being reused six times, the COD-immobilized hollow fiber retained more than 80% of the activity.