Nitrogen plasma was used to amino-functionalize polystyrene surfaces, which were further modified via the selective introduction of polyamines suitable for the immobilization of biological compounds. This chemical modification was carried out using a multifunctional amine compound linked to glutaraldehyde, leading to the formation of hyperbranched structures at the surface. Up to three generations of branched polymers at the polystyrene (PS) surface were created by successive addition of the functional compounds. Amine functions introduced at the surface were labeled with 2,3,4,5,6-pentafluorobenzaldehyde and analyzed by X-ray photo, (XPS), confirming the successful electron spectroscopy attachment of each generation of branching. Finally, bovine serum albumin and trypsin were immobilized on N-2-Plasma-treated PS modified with different amounts of branched graft polymer and found to remain bioactive after immobilization. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 2701-2709, 2009