Penicillin G acylase (PGA) has been immobilized onto nylon membranes grafted with methylmethacrylate (MMA) or diethyleneglycoldimethacrylate (DGDA) monomers by means of -radiation. Hexamethylenediamine (HMDA) has been used as spacer between the grafted membranes and the enzyme. Glutaraldehyde (GA) was used as crosslinking to couple the enzyme to the HMDA. The catalytic membranes so prepared were studied as a function of pH and temperature of the solution containing the substrate. The membranes showing the best characteristics were the ones grafted with DGDA. The dependence of the behavior of these membranes on several experimental conditions was studied, i.e., the temperature and duration of the aminoalkylation process, spacer concentration, the glutaraldehyde concentration and the enzyme concentration. The experimental conditions giving the best performance of the catalytic membranes have been deduced. The time requested to obtain 50␘f substrate conversion, i.e., hydrolysis of cephalexin, has been studied as a function of its initial concentration.