N,N,N',N'-Tetramethyl(succinimido) uronium tetrafluoroborate is proposed as a reagent of choice for the activation of carboxyl groups and formation of N-hydroxysuccinimide esters on polymers. Unlike conventional methods which generate unstable gels, the reaction is appropriate for hydroxy-containing resins like Sepharose, cellulose, and dextran. The yields of activation and subsequent coupling capacity for ligands and proteins are very high. The respective columns can be used for affinity chromatography and immobilization of proteins.