In this paper, tests of an optimized membrane-stirrer geometry for bubble-free aeration of a plant cell suspension culture are described. Cell attachment and clogging of a previously described system [Piehl et al. (1988) Appl Microbiol Biotechnol 29:456-461] led to the development of a new stirrer. The volumetric oxygen transfer capacity has been measured in aqueous medium. The mass transfer coefficient, k(1)a, was 3.75 h(-1) at 25 degrees C and at a stirrer speed of 34 rpm. The overall oxygen transfer capacity was investigated with a suspension culture of Aesculus hippocastanum It was shown that the oxygen mass transfer was sufficient even at the maximum biomass of 10-12 g dry weight/l, which was obtained by using this system. Furthermore, special attention was given to medium components like C and N sources, to avoid growth limitation due to a shortage of nutrients.