This paper investigates the formation of concentration polarization in microfiltration (MF) and ultrafiltration (UF) of bovine blood and albumin solutions on polymeric membranes. The membranes are submitted to stepped pressure increments from an unpolarized condition to a highly polarized state while the instantaneous permeate flux variation is monitored by a medical type electromagnetic flowmeter. When a 0.5 mu m pore MF membrane is used, the flux reaches a peak in 0.5 s and decays exponentially to its equilibrium level with a time constant of the order of 1 s. The limiting flux occurs in about 2-3 s after filtration of 35 cm(3)/m(2) of plasma when the return mass flux of red cells towards the bulk solution matches the incoming flux dragged by filtration. The time variation of the return mass flux is computed from a model based on a time dependent mass balance at the membrane. The specific resistance of the cell layer is of the order of 6 x 10(14) m/kg at a pressure of 30 kPa. When another pressure increment occurs in the polarized state the red cell layer adjusts its resistance to maintain the same equilibrium permeate flux by further compression without additional deposition. When the same experiments are repeated with a UF membrane, the permeate flux follows a similar pattern but the time constant of the exponential decay is much longer (36 s for blood and plasma, 16 s for albumin) and equilibrium flux is reached after 2 min for blood, 30 s for albumin. These data confirm that proteins play a dominant role in the establishment of concentration polarization in UF even though they occupy a much smaller volume fraction than the red cells. The contribution of red cells to the overall filtration resistance is about 20%. But the specific deposit resistance of the protein layer is estimated to be of the order of 6 x 10(15) m/kg at a pressure of 130 kPa, much higher than for red cells. This protein layer is less compressible and further increase in pressure results in additional thickening of the deposited layer. These data explain why 1 Hz flow and pressure pulsations can disturb the red cell deposited layer in plasmapheresis and increase the plasma permeate flux while the same pulsations are inefficient in hemofiltration.