Critical fluxes were determined by constant flux ultrafiltration (UF) experiments under laminar flow conditions. The experiments were performed by using hydrophilic C30G and hydrophobic GR51 ultrafiltration membranes and dilute myoglobin solutions and baker's yeast suspensions as model colloids. Solution concentration, pH and cross-flow were investigated. The critical flux increased with increasing flow velocity and decreasing solute concentration. The regenerated cellulose C30G membrane exhibited hi-her critical fluxes than the polysulphone GR51 membrane. The highest critical flux was obtained at pH 8 in the presence of repulsive electrostatic forces between the molecules and the surface of the membrane and the lowest at the isoelectric points of the colloids. In the case of baker's yeast below the critical flux, the flux was about the same as the pure buffer solution flux showing a strong form of the critical flux. This also occurred with the C30G membrane at low concentrations of myoglobin except at pH 6 when a weak form of the critical flux was measured. With the GR51 membrane, the permeate flux deviated from the pure buffer solution flux even at the lowest fluxes.