An enzyme membrane system was constructed by immobilizing trypsin onto chitin and ultrafiltration set-up was employed to investigate the activity of thus prepared enzyme membranes. Benzoylarginine ethyl ester (BAEE) and casein were used as substrates. Using BAEE as a substrate, the dependence of conversion on pH was examined under flow condition (ultrafiltration). The results showed that the optimum pH for the enzyme membrane was shifted from 7.8 for native enzyme toward more alkaline pH 9.0. The shift was assumed to be caused by the localized high concentration of enzyme molecules and of the released carboxylic acids during the hydrolysis of substrate. In order to measure the "figure of merit" the reaction rates with a membrane were measured under the flow and batch conditions. The rates were calculated from the conversions of substrate, when the conversions for both flow and batch conditions were equal. The results obtained with BAEE as a substrate gave the figure of merit 5.67, indicating that reaction rate for flow system was five times larger than that for batch system. This result implies that in the forced flow of substrate solution through an enzyme membrane, diffusion limitations could be eliminated by exposing all the enzyme molecules in the membrane to incoming substrate molecules. When casein, a high molecular weight substate, was used this tendency was expected to be more pronounced. In the case of casein conversion was expressed by total uv absorption (△E) and the figure of merit was determined by comparing the time periods required to reach the same uv absorption, i.e., the same extent of reaction. The results showed that the figure of merit was 10.3, and then figure of merit for casein was two times larger than that for BAEE.