The performance of membranes used in the separation of solution mixtures containing proteins can be substantially limited by aggregation and deposition inside membrane pores and on their surfaces. in this study flux, stepping and constant flux operation have been used in conjunction with varying electrolyte concentrations and pHs in the microfiltration of aqueous solutions of bovine serum albumin. Experiments were performed to investigate the hydrodynamic and solution conditions during the incipient fouling stage which lead up to eventual cake formation. Results from flux stepping showed that an increase in wall concentration coincides with the onset of an apparent critical flux. This is followed by a time lag before an increase in observed rejection is exhibited. Sub-critical constant flux operation showed that there exists an aggregation and deposition time lag after which the membrane suddenly experiences a rapid increase in hydraulic resistance due to protein aggregates blocking a majority of membrane pores. These incipient fouling conditions were shown to be dependent on pH and ionic strength and were concluded to be the product of a balance between electrostatic, solubility and hydrophobic effects which were manifested in protein-protein and protein-membrane interactions.