In the production of S-(+)-ibuprofen (S-IP) from racemic 2-(4’-isobutylphenyl)propionitrile (IP-CN) by cells of Acinetobacter sp. AK 226, the enzymatic reaction was inhibited by the S-IP produced, and cell activity was decreased to 20% when cells were incubated with 30 g/l of S-IP for 3 h. A method was devised to avoid inhibition and inactivation by the product. An ultrafiltration (UF) membrane was more suitable than a microfiltration (MF) membrane for the selective removal of S-IP from the reaction mixture because more IP-CN could be filtered during microfiltration. When the reaction was run in a batch reactor at the initial concentration of 100 g/l IP-CN, cells were inactivated completely before conversion reached 30%. On the other hand, 26% of cell activity was left even after conversion reached 35% when the batch reaction and UF were combined to remove S-IP from the reaction mixture continuously in order to maintain a concentration of 10-20 g/l S-IP. When 400 g/l of nitrile was fed continuously and the S-IP concentration was maintained at 10 g/l by a computer-controlled system, the reaction rate was constant except for the initial decrease, and a 34% conversion was obtained.