Polysulfone was chemically modified by a metalation reaction which could be further reacted with glycidyl-4-oxohexylether. Thus, the final polymer contained a reactive epoxy group with a 7-atom distance from the polymeric backbone. From this modified polysulfone, asymmetric ultrafiltration membranes were cast conventionally and iminodiacetic acid was bonded to the epoxy group. The reaction was proved by electrokinetic measurements. The membranes, now with chelating properties, were complexed with copper ions. By this method a high-performance affinity matrix was obtained which was used for the adsorption of various amino acids. Using histidine the complexation constant was determined to log K = 4.34 (phosphate buffer, pH 7, c(NaCl) 1 M). The affinity membranes were used for concentrating a dilute histidine solution (c = 4 mumol/l) as well as for the separation of a solution consisting of four amino acids.