Conventional, crossflow microfiltration systems rely on high liquid crossflow velocities to generate shear at the liquid-membrane surface. Shear is necessary to maintain acceptable permeate fluxes and solute transmission especially at high biomass concentrations. In contrast, vibrating membrane filtration (VMF) technology uses mechanical energy generated by vibration to create these shear fields thus decoupling shear from the liquid crossflow velocity. This allows the maintenance of permeate fluxes and solute transmission at much lower crossflow velocities and transmembrane pressures. In this work, we report experimental data for a commercial VMF system (PallSep PS10) for protein recovery from a model biological feed stream containing 200-500 g l(-1) Saccharomyces cerevisae wet weight and 0.75 g l(-1) bovine serum albumin (BSA). For operation in total recycle mode, basic permeate flux and transmission characteristics of the VMF system are reported for operation at these high biomass loadings as a function of the frequency and peak to peak amplitude of vibration. At a biomass concentration of 500 g l(-1), for example, flux and transmission levels of 45 1 m(-2) h(-1) and 67% (w/w), respectively, were maintained over extended operating periods. For operation in concentration mode, a theory is proposed that allows prediction of the maximum solids handling capacity of the VMF unit. We show how this value, and hence the maximum volumetric concentration factor that can be achieved, is influenced by gap width between the membranes. The model is then verified experimentally using S. cerevisae feedstreams showing that the predictions are accurate within 14%. The influence of increasing membrane gap width is also shown using real industrial fermentation broths from Bacillus and Aspergillus cultures. The results for both real and model system studies demonstrate the benefits of VMF systems in terms of both permeate flux and protein transmission especially when operating at high solids loadings. (C) 2003 Elsevier B.V. All rights reserved.