High-performance membrane chromatography (HPMC) is a very effective chromatographic method in which all the mobile phase flows through the separation medium. The effects of process variables such as concentration of displacement agent, flow-rate and gradient slope on HPMC separations in the ion-exchange, hydrophobic interaction and reversed-phase modes were studied using model protein mixtures. The basic relationships characterizing column HPLC also apply in HPMC. Whereas the efficiency of the HPMC membrane does not depend on flow-rate, the resolution increases with increasing gradient volume. Separations obtained with a continuous linear gradient were used for the design of a stepwise gradient profile which decreases the consumption of both time and mobile phase in separations of proteins. According to calculations, the protein diffusivity enhanced by the convective flow through the membrane is about four orders of magnitude higher than the "free" diffusivity of the protein in the stagnant mobile phase located in the pores of a standard separation medium. This considerably speeds up the process and improves the efficiency of the separation.