The effects of varying morphology and surface chemistry on protein fouling of microfiltration membranes were investigated. In part I of the study, on the effects of varying morphology, results show that 0.2 mu m track-etched polycarbonate (PC) membranes internally foul, with external fouling becoming the dominant means of fouling only at later times. A 0.2 mu m cellulose acetate (CA) membrane showed only internal fouling, while 0.2 mu m polysulfone (PS) and polyvinylidene fluoride (PVDF) membranes showed only external fouling. It is hypothesized that the low surface porosities of the PS and PVDF membranes lead to almost immediate external fouling, while the higher surface porosities of the PC and CA membranes allow for a significant period of time for internal fouling to occur. For each membrane, protein transmission remained constant or only slightly decreased during internal fouling, while a significant loss of protein transmission was observed during external fouling. In part II of the study, on the effects of various surface modifications, results show that surface-modified polyethylene and polypropylene membranes have lower initial fluxes than the unmodified membranes. However, the hydrophilic modified membranes demonstrated comparable final fluxes and lower percent flux declines than the unmodified membranes. The azlactone modified membranes showed very low long-term fluxes and large decreases in permeate protein concentration due to efficient protein binding. Again, protein transmission remained constant or only slightly decreased during internal fouling, while a significant loss of protein transmission was observed during external fouling.