Poly(ethylene glycol) (PEG) has been grafted to polystyrene using branched poly(ethylene imine) (PEI) as an anchoring polymer. A dense grafting of PEG chains is obtained if a PEG-PEI adduct is first prepared by treatment of PEI with PEG epoxide and the adduct subsequently adsorbed on an oxidized polystyrene surface. Prepared in this way the PEG-PEI adduct is irreversibly attached to the surface with the PEG chains oriented towards the aqueous phase, as shown by ESCA and ellipsometry. This dense PEG coating is highly effective in preventing adsorption of fibrinogen and IgG, as well as adhesion of the oral bacterium Streptococcus mutans. Somewhat surprisingly, the degree of PEG branching, as well as the PEG molecular weight, did not markedly influence protein rejecting properties. The water contact angle on polystyrene decreased in the order: untreated polystyrene > oxidized > PEI treated > PEG-PEI treated. Block copolymers of poly(ethylene glycol)/poly(propylene glycol) type attached to polystyrene showed temperature dependent protein rejecting properties. At temperatures well below the cloud point of the polymer in water the surface is protein repellant whereas around and above the cloud point protein adsorption occurs to a considerable extent. The block copolymer layer expands and contracts with temperature variations, as can be monitored by ellipsometry.