Aim of the present work is the experimental analysis of a continuos flow membrane bioreactor with immobilized chymosin for the hydrolysis of milk k-casein, one of the steps in the cheese production process. Chymosin has been immobilized on the bioreactor membrane by ultrafiltering under pressure its solutions, continuously fed to the system in a total recycle configuration. Bioreactor performances are evaluated in terms of two different parameters: chymosin activity and its release. The former, indirectly, measures the amount of k-casein converted into reaction products (para-k-casein and GlycoMacroPeptide, GMP); the latter quantifies the amount of biocatalyst that is removed from the membrane by the milk flowing stream. A theoretical approach, based on the boundary layer theory, has also been attempted to estimate the dependence of chymosin release upon the wall shear stress and on the back-diffusion phenomena towards the milk bulk. The experimental analysis of bioreactor behavior has been carried out, searching for the operating conditions and system configurations that maximized chymosin activity and minimized its release. Particular attention has been devoted to the selection of the membrane material: PolySulphone membranes offered the best performance during experiments, as compared to those of PolyPropilene and PolyVinilDeneFluoride. The system configuration that gave best results, both in terms of enzyme activity and of its release, was based on a continuous milk recycle to the bioreactor. Enzyme losses were confined to 5-10 mg/l range, whereas its activity always attained values high enough (>2 g/l) to promote the coagulation in the next heating step of destabilized milk. During the experiments, an optimal value of the recycle ratio, was also found.