Bovine serum albumin (BSA) solutions of 0.1 g/liter and 1.0 g/liter were filtered through 0.05 mum and 0.2 mum polycarbonate microfiltration membranes using a stir cell. Total resistance versus time and permeate concentration versus time curves and scanning electron micrographs were generated. From these, together with comparing the results to internal and external fouling models, several different fouling mechanisms are hypothesized. For 0.1 g/liter concentration and 0.2 mum pore diameter, the fouling occurs at the mouths of the pores, slowly closing off pore entrance areas while allowing complete transmission of the protein for a period of time. Eventually, the pores become so constricted that protein transmission decreases and a layer of rejected protein forms on the external membrane surface. For 0.1 g/liter concentration and 0.05 mum pore diameter, fouling immediately closes off pores, and permeate concentration decreases quickly. For 1.0 g/liter concentration and 0.2 mum pore diameter, the fouling allows for nearly complete transmission of proteins for the entire length of the experiment, unlike the more dilute case of 0.1 g/liter. Since the diameters of BSA molecules are more than an order of magnitude smaller than the pore radii, it is believed that the proteins form aggregates which accumulate on or just below the top surface of the membrane, depending on their size relative to the pore openings.