Microporous nylon 6,6 membranes were modified by grafting poly(2-acrylamido-2-methyl propane sulfonic acid (poly(AMPSA)) on the membrane surface. The poly(AMPSA) grafting on the membrane surface was confirmed by spectroscopic characterization. Several measures of the microfiltration membrane porosity indicated that the morphology was not significantly affected by the grafting procedure. Buffer fluxes through the modified membrane were very sensitive to the solution ionic strength (I) and also to the solution pH. The order of magnitude variation in buffer fluxes at different ionic strengths can be explained by charge-shielding of the poly(AMPSA) chains; at low I values, the poly(AMPSA) chains experience mutual repulsion, swell and hinder transport through the pores. Conversely, as I increases, charge-shielding allows the poly(AMPSA) chains to take a more compact conformation. The flux and rejection behavior during the protein (BSA) transport was consistent with this hypothesis and was also dependent on protein-membrane electrostatic interactions.