The electrophoresis of bovine serum albumin (BSA) and bovine haemoglobin (BHb) through microfiltration membranes has been investigated using an apparatus consisting of two well-stirred chambers separated by a microfiltration membrane, with electrode compartments at each end. The electrophoretic mobilities of the proteins were determined under conditions of zero concentration driving force and with negligible adsorption on to the membranes occurring during an experiment. Using a track-etched membrane of 9% porosity and 0.2 mu m pore size, the initial mobilities of both proteins were close to published free-solution values at the same pH, after correction for the ionic strength of the buffer; this was so because the effect of reduced free area in the membrane was compensated by a corresponding increase in the potential gradient. Similar values were also obtained for BHb with four other microfiltration membranes of 70-75% porosity and 0.10 to 0.22 mu m pore size, made from various materials; however BHb mobilities 26% lower were obtained with a membrane of 0.025 mu m pore size. Data were also obtained for mixtures of the two proteins; these indicated, as expected, that interactions occurred when they were oppositely charged, changing the apparent mobilities from the free solution values. The single protein data were interpreted using a phenomenological model, which incorporated the effects of electroosmotic flow resulting From membrane surface charge, hydrodynamic drag, pressure-driven back-flow and back-diffusion. The mixed protein data were modelled using the Maxwell-Stefan relations, with promising results.