Microfiltration is an attractive method for the clarification of fermentation broths provided that sufficiently high permeate flux and product transmission can be achieved. Several polyelectrolytes were screened for flocculated particle size and permeate flux enhancement in unstirred and stirred dead-end filtrations. Larger flocculated particles were formed by flocculation with polyelectrolytes of longer-chain length. Increase in particle size of the feed generally resulted in an increase in the permeate flux, but the largest particle size did not result in the greatest permeate flux enhancement. In crossflow filtrations, long-term permeate flux improvements occurred despite complete disruption of the floc in the recycled feed and the extent of the improvements were dependent on both the polyelectrolyte dose and crossflow rate. The size of the particles comprising the cake layer closely resembled that of the particles in the initial flocculated feed, indicating that the particles that initially deposited on the cake surface avoided disruption during filtration. Additionally, a decrease in subtilisin rejection was observed with filtration of the flocculated feed. Thus, flocculation with cationic polyelectrolytes increased permeate flux and reduced product rejection in the microfiltration of a complex industrial fermentation broth. (C) 2008 Elsevier B.V. All rights reserved.