In the enzymatic production of antibiotics, membranes can be used for different purposes. Since the enzyme is a cost-determining factor, membrane bioreactors in which the ultrafiltration membrane's sole purpose is retention of the 'free' enzyme and recycling it to the reaction vessel, were investigated as an alternative for the current immobilised-enzyme based process. The membrane bioreactor was tested for two reactions, the production of one of the substrates for the antibiotic synthesis, and the actual enzymatic synthesis reaction. The first process could be improved considerably if the pH regulation took place right after the membrane, therewith creating a small loop that prevented direct contact of the enzyme with high pH, for which it is very sensitive. For the latter process, a big advantage was that diffusion limitation, which reduces the yield of the process with immobilised enzyme, does not take place. Besides, options for continuous operation became available. Away from application during the enzymatic reactions, (nanofiltration) membranes were useful for downstream processing of one of the substrates for antibiotic synthesis. Some of the tested membranes showed separation behaviour, which was strongly pH dependent, therewith opening new options for isolation of the desired component.