The covalent immobilization of a commercial preparation of cellulase on a reversibly soluble-insoluble enteric polymer Eudragit S-100 by carbodiimide coupling was carried out. The characteristics of covalent Eudragit cellulase were evaluated using Fourier transform infrared (FTIR) spectra, circular dichroism (CD) spectra, and fluorescence spectra. FTIR, CD, and fluorescence measurements also revealed that the cellulases were covalently bonded to the supports. Covalent Eudragit cellulase had binding efficiency of 81.08% which was higher than the noncovalent Eudragit cellulase 56.83%. The relative activity of the native cellulase and covalent Eudragit cellulase increased and reached the maximum (at pH 5.0, 50 degrees C) and then decreased with further increases in pH and temperature. The covalent Eudragit cellulase shows higher stability especially at higher pH and temperature. The K-m value of covalent Eudragit cellulase (4.78 g.L-1) was decreased compared to that of the native cellulase (2.89 g.L-1). The affinity of the cellulase to its substrate was increased when it was immobilized on Eudragit S-100.