The effect of viscosity and temperature on the flux from a stirred ultrafiltration cell was studied. Fluxes of water, sucrose solutions, and sucrose solutions containing bovine serum (nonpermeating species) were measured at various temperatures. The effect of sucrose concentration and temperature on flux was explained by determining the effect of these two variables on the bulk viscosity and the diffusivity of the nonpermeating species, and then correlating flux by the equation Sh =A(Re)α(Sc)1/3, The flux at various temperatures could also be adequately estimated from changes in viscosity alone.