Rates of ultrafiltration of bovine blood of various hematocrits were measured in ultrafilters of hollow-fiber type. The results were analyzed by the gel-polarization model. The mass transfer coefficientk for the back-diffusion of protein molecules away from the gel layer, which controls the ultrafiltrate flux, increase with the cube root of the shear rate at the membrane surface. At a given shear rate, it increases and then decreases with increasing hematocrit. A working equation for predictingk is proposed.