A specially designed bioreactor including an axial microfilter for cell retention was evaluated for continuous-flow operation with selected liquid media as controls and in aerobic cultivations of Saccharomyces yeasts. In the initial tests, performance characteristics such as filtration rates and cell accumulation were assessed as a function of filter rotational speed, operating pressure, cultivation time and microfilter type (i.e. membrane or porous metal). The bioreactor did not perform satisfactorily when viscous extracellular polymer was present in the liquid. In the continuous-flow culture enabling cell retention, Saccharomyces cerevisiae yeast cell concentrations were enhanced by as much as 16-fold over ordinary batch growth. Concomitant filtration rates were stable over operating times of up to 130 h and hence were independent of the cell concentration. The maximum steady-state flux was enhanced at rotational speeds up to 400-700 rpm ranging from 22 to 42 L m(-2) h(-1). Higher rotation rates offered no further improvements. The maximum stabilized flux was independent of operating pressure. Pressure increases caused momentary flux improvements, which rapidly declined and eventually restabilized. (c) 2006 Society of Chemical Industry.