it is suggested that the adoption of efficient counterflow cascades may accelerate the continuing encroachment of membranes on chromatography for downstream processing of biologicals, and a specific numerical example is provided to demonstrate their effectiveness. Emphasis is on three-stage ideal cascades, and it is shown that one may begin using the traditional batch operating mode. Conversion to continuous operation is then both simple and straightforward. Membrane cascades are the only means so far available for true continuous downstream processing of therapeutic proteins, which is a natural extension of the continuous upstream processes already beginning to be used for industrial production. Membranes are also attractive for larger entities such as plasmids or viruses whose low diffusivities can severely limit use of chromatographic processes.