Factors affecting the performance of crossflow filtration were investigated with a thin-channel module and yeast cells. In crossflow filtration of Saccharomyces cere- visiae cells cultivated with YPD medium (yeast extract, polypeptone, and dextrose) and suspended in saline, a steady state was attained within several minutes when the cell concentration was low and the circulation flow rate was high. The steady-state flux and the change in flux during the initial unsteady state were explained well by conventional filtration theory, with the amount of cake deposited and the mean specific resistance to the cake measured in a dead-end filtration apparatus used in calculation. When the circulation flow rate was lower than a critical value, a part of the channel of the crossflow filtration module was plugged with cell cake, and thus the steady-state flux was low. In crossflow filtration of suspensions of commercially available baker's yeast, the flux gradually decreased, and the flux after 8 h of filtration was lower than the value calculated by filtration theory. Fine particles contaminating the baker's yeast was responsible for the decrease. A similar phenomenon was observed in crossflow filtration of a broth of S. cerevisiae cells cultivated in molasses medium, which also contains fine particles. The YPD medium, which does not contain such particles, had no effect of the permeation flux during crossflow filtration.