The effects of transmembrane pressure (TMP) and trans-cartridge pressure drops on the concentration of protein precipitate suspensions are evaluated in a hollow fibre device. A sharp decline in the filtrate flux occurs above a critical concentration, the value of which depends on the hydrodynamic operating conditions, and this is shown to coincide with the system entering a state of pressure-independent flux, commonly known as "gel polarisation". The effect of TMP, cross-flow velocity and concentration on the onset of pressure independence are examined in more detail in a series of filtrations carried out at constant concentration. Increasing cross-flow velocity enhanced fluxes for a given TMP in accordance with the standard film model, but increasing concentration was found unexpectedly to enhance fluxes at relatively low concentrations. Two possible explanations for this are discussed; one depends on the interation of the large protein precipitate a function of wall shear rate is compared with available theoretical predictions which are based on the solid and the dissolved protein considered separately and it is shown that neither approach can alone describe the mixed system under investigation.