The effect of interfacial chain density of poly(ethylene glycol) (PEG) on the adsorption of serum proteins was investigated by in situ ellipsometry. For terminal covalently grafted PEG of molecular weight 5000 an increased grafting density results in decreased serum protein adsorption. At high interfacial chain density (approximate to 0.1 chain/mm(2)), efficient protein rejection was observed, irrespective of the coupling chemistry used. Strongly adsorbed PEG-containing polymers behaved similarly to covalently attached PEG regarding inhibition of protein adsorption, independent of the nature of the underlying surface. The results are discussed in relation to the importance of the protein size in protein rejection by PEG coatings.