Flocculation of yeast cells prior to microfiltration leads to improved permeate flux. A number of commercially available cationic polyacrylamides from Cytec Industries (Stamford, CT, USA) were tested. These flocculants had different molecular weights and charge densities. The optimum flocculant dose and stirring conditions were determined for each polymer. The particle size distribution before flocculation and after flocculation and microfiltration was determined. The permeate flux depended strongly on the particle size distribution. Very small micron and submicron sized particles plug the pores of the membrane leading to decreased permeate flux. Very large flocs, 500 mum or larger plug the lumen of the hollow fibres again leading to a decrease in permeate flux. The optimum particle size is about 100-200 mum. The permeate flux increased with increasing molecular weight and charge density (percentage of monomer units that are charged) of the flocculant.