Flat sheet polypropylene microfiltration membranes were successfully graft-modified with acrylic acid using the macroinitiator method. The surface modification induced a change in the course of fouling. Unmodified membranes showed the strong form of critical flux during filtration of protein solution, while for the surface modified membranes the weak form of critical flux was determined. Although the critical flux could be shifted towards higher values (from ca. 20 l/m(2) h to > 40 l/h(2) m) by the surface modification, the permeability decreased by a factor of 6.