As part of the method for large-scale preparative electrophoresis across membranes of controlled porosity, we show that successive amounts of the leading component of a mixture migrating across a thin membrane can be collected by 'reflux electrophoresis'. This consists of a series of cycles, in each of which the forward phase is stopped before the trailing components can emerge, the leading fraction is collected and the membrane cleared by reversing the current before commencing the next cycle. The reflux principle is demonstrated by separations based on size or on charge differences of protein molecules.