This work investigates the fractionation of simulated mixtures of lysozyme/ovalbumin and lysozyme/myoglobin in a vortex flow ultrafilter. The operating parameters for the separations were identified from transmission studies conducted previously for each of these three proteins [3]. At low transmembrane pressure and high membrane rotation speed, the ultrafiltration characteristics of a dilute protein mixture were virtually identical to those of its individual components. The selectivity was controlled primarily by the extent of polarization of the smaller, preferentially transported species (lysozyme). Thus, high separation factors were obtained under operating conditions favoring lysozyme buildup at the membrane surface. It was further observed that better separation occurred at low fluxes (corresponding to low applied pressure) thereby indicating that the throughput and degree of resolution are mutually exclusive.