Penicillin G was continuously extracted from an aqueous buffer solution at pH 5 across a membrane of Amberlite LA-2 isodecanol kerosene solution in a Hoechst-Celanese hollow fibre membrane module, and re-extracted from the organic phase across a membrane by phosphate buffer solution at pH 8 in a second Hoechst-Celanese hollow fibre module. The penicillin G in the stripping phase was converted to phenylacetic acid (PAA) and 6-aminopenicillanic acid (6-APA) by free and immobilized penicillin G amidase (PA) in phosphate buffer solution, respectively. PAA was removed from the reaction mixture by electrodialysis. For this purpose sandwich and composite bipolar membranes were compared, based on the electrodialysis of acetate and citrate. The electrodialysis of penicillin G, PAA, 6-APA and phosphate were investigated under various operational conditions to determine the optimal conditions for the retention of 6-APA and the removal of PAA from the reaction mixture at optimal pH value for the stripping of penicillin G and its conversion to PAA and 6-APA. PAA was removed from the educt cycle by electrodialysis into the product cycle. The pH in the educt cycle was reduced to a value of 6 by permeation of phosphate into the product cycle. Free and immobilized PA, respectively, and phenylglycine ethyl ester (PGE) were added to the educt cycle of the electrodialyser and the latter reacted with 6-APA to form ampicillin.