Microporous polysulfone hollow fibers were modified by reacting terminal phenol groups with ethylene glycol diglycidyl ether (EGDGE) to produce terminal epoxy groups. These were used to covalently link hydroxyethyl cellulose (HEC) polymers to the membrane surface. Further epoxidation of the HEC and reaction with hexane-diamine produced amine-terminated leashes on the microporous fibers. Recombinant protein A was linked to the amine functions and the resulting affinity membranes were evaluated for IgG capacities. The EGDGE reactions were studied under both acidic and basic catalysis conditions. IgG capacities of the acid-catalyzed reaction products reached as high as 20 mg/ml bed volume. Comparisons of equilibrium and dynamic capacities show that dynamic capacities are somewhat lower than the equilibrium capacities.