Penicillin G hydrolysis in a compartmentalised electro-membrane reactor with penicillin G acylase immobilised in a polyacrylamide gel Jab was studied. Penicillin G solution was fed to the substrate compartment stacked to one gel slab surface and reaction products were colleted in the product compartment adjacent to the other slab surface. An electric field perpendicular to the slab surface was imposed to the reactor by a couple of platinised titanium electrodes. The enzyme activity of the gel slab was varied from 30 to 150 U cm(-3) and density of the electric current passing through the slab (5% polyacrylamide, dimensions 40 mm x 40 turn x 2.7 mm) was varied from -1250 to +625 A m(-2). The current was considered as negative when electrophoretic migration augmented penicillin G transport from the substrate compartment to the product one. The reactor was operated in recirculation (batch) mode. The initial penicillin G concentration was 2% (w/v). Substantial increase of the rate of the penicillin G hydrolysis and of the volumetric slab productivity was observed both at negative and positive electric currents applied to the reactor. Fast replenishment of the consumed substrate was the reason of intra-slab reaction acceleration under the negative currents. The positive currents resulted to a close-to-optimum intra-slab pH value and consequent increase of the reaction rate. The positive currents, however, yielded only very poor separation of the reaction products from the unreacted substrate. Good separation was achieved when negative current of the density of -1250 A m(-2) was applied. The volumetric productivity of the slab at this current density was about 240 mol of the penicillin G hydrolysed per litre of the gel per one hour of reaction time. Temperature rise in the reactor due to the Joule heating was acceptable at all current densities. (C) 2003 Elsevier Inc. All rights reserved.