Cross-flow ultrafiltration of protein mixtures continues to be more attractive in theory than achievable in practice due, in large part, to the rapid decrease in permeation efficiency that occurs when proteins come in contact with most membranes. This study takes a fresh look at the use of a oscillatory flow across corrugated membranes; a process reported to improve permeation efficiency through vortice-induced secondary mixing. Results of this empirical study demonstrate that, with naturally hydrophilic membranes, the primary positive effect of oscillatory flow on permeation efficiency is the result of the oscillatory pressures generated by oscillatory flow. Permeation efficiencies approaching 100% (i.e. permeation rates with protein solutions equal to those of pure water) are maintained for operation times of more than two weeks in smooth feed channel devices when oscillatory flow is used with cellulosic asymmetric ultrafiltration membranes.