The vibrating membrane bioreactor (VMBR) system facilitates the possibility of conducting a separation of macromolecules (BSA) from larger biological components (yeast cells) with a relatively high and stable macromolecular transmission at sub-critical flux. This is not possible to achieve for a static non-vibrating membrane module. A BSA transmission of 74% has been measured in the separation of 4g/L BSA from 8 g/L dry weight yeast cells in suspension at sub-critical flux (20L/(m(2) h)). However, this transmission is lower than the 85% BSA transmission measured for at pure 4g/L BSA solution. This can be ascribed to the presence of extracellular polymeric substances (EPS) from the yeast cells. The initial fouling rate for constant sub-critical flux filtration of unwashed yeast cells is 3-4 times larger than for washed yeast cells (18(mbar/h)/5(mbar/h)). At sub-critical flux, an EPS transmission of around 32% is measured for a pure yeast cell suspension. Thus, EPS and BSA are "competing" in being transmitted which might explain the lowered BSA transmission in the presence of yeast cells. Additionally, EPS heavily foul the membranes, leading to a 86% permeability drop and a fouling resistance 6 times larger than the membrane resistance after 5 1/2 h of constant sub-critical flux filtration (20 L(m(2) h)) of pure 8 g/L dry weight yeast cell suspensions. Thus, the addition of hydraulic resistance caused by EPS might also explain the lowered BSA transmission, in the presence of yeast cells, since the membrane pores might be narrowed or partly blocked. EPS is, furthermore, able to cause a relatively large permeability drop even on a membrane module pre-fouled by EPS. (c) 2008 Elsevier Ltd. All rights reserved.