alpha -Chymotrypsin was immobilized in inorganic microcapsules made of calcium silicate. In order to evaluate the performance of immobilized alpha -chymotrypsin, the rate of hydrolysis of N-acetyl-L-tyrosine ethyl ester (ATEE) and the thermostability of alpha -chymotrypsin were examined. The hydrolysis of ATEE by both free and encapsulated alpha -chymotrypsins apparently obeyed Michaelis-Menten kinetics. The apparent Michaelis constant in the encapsulated system nearly equated that in the free enzyme system. The extent of decrease in maximum velocity by encapsulating alpha -chymotrypsin was much smaller than that by encapsulating lipase, suggesting that the hydrophilicity of the substrate is a key factor in the diffusion process through the wall due to the hydrophilic surface of the microcapsules. The thermostability of cc-chymotrypsin was considerably enhanced by the microencapsulation. The half-life of encapsulated alpha -chymotrypsin was 23 times as long as that of the free one.