AbstractBatch and flow recirculation cells were used to study the properties of high‐flux ultrafiltration membranes with different macromolecular solutions. At low pressures, solutions of completely retained macromolecular solutes have a flux which is approximately the same as the flux of pure solvent. At higher pressures, the solution flux levels off. The flux, at the leveling‐off period, is approximately inversely proportional to the solution concentration. In this plateau region the flux increases with temperature and agitation of the solution but decreases with time. These results are explained by the formation of a gel layer on the membrane surface during the filtration of macromolecular solutions. In ultrafiltration, in contrast to dialysis and GPC, a linear polymer penetrates the selective barrier more readily than does a globular protein of the same molecular weight. The difference may arise from the liquid shear stresses within the barrier medium due to the movement of fluid relative to the pore walls, which is large only in ultrafiltration. Also, retention of polymers was found to decrease with pressure and to increase with agitation of the solut
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