An unsteady-state model is developed to predict the feed surface and permeate concentration for ultrafiltration of macromolecules. A priori knowledge of the diffusion coefficient through the membrane is however necessary. It is shown that the model is capable of predicting the feed surface concentration once the operating conditions are fixed and does not make an implicit assumption of constant feed surface concentration. It is further shown that for a certain range of system parameters, a negative rejection (based on the feed concentration) is possible at steady state.