Since both phases of aqueous two-phase systems are electrically conductive, application of electric fields in these systems gives rise to electrokinetic mass transfer of charged species. We have shown that an aqueous two-phase system can be used as a medium for electrophoretic separation with the liquid-liquid interface providing stability against convection and facilitating product recovery. Proteins have been directed into either the top or bottom phases of polyethylene glycol/dextran aqueous two-phase systems using 20-50 V/cm electric fields perpendicular to the phase interfaces. Binary protein mixtures have been separated in both batch and continuous modes by operating between isoelectric points and directing oppositely charged proteins into separate phases. Preliminary results on focusing E. coli cells into one phase while directing a proteinaceous product into the other phase indicate that two-phase electrophoresis may prove useful for commercial-scale recovery of proteins from fermentation broth. Recent studies in our laboratory indicate that the electrostatic potential profile near the interface of conducting liquid-liquid systems can influence electrophoretic migration of charged species across the interface. We have observed that phase systems with high Donnan potentials hinder electrophoretic mass transfer across liquid-liquid interfaces, whereas phase systems with low Donnan potentials do not.