Protein adsorption occurs on the surface of biomaterials when they are exposed to physiological environments. The protein adsorption layer induces severe biological responses, including cellular reactions. Protein adsorption layers are formed mainly by two distinct processes: monolayer adsorption of proteins on the surface and a subsequent additional adsorption on the first layer to form a multilayer. Therefore, evaluating the first protein adsorption is important to understand the biological responses on the surface of materials. In this study, we applied the atomic force microscopic (AFM) technique to directly measure the adhesion force of proteins against the surface (i.e., the interaction between proteins and surface). We also prepared hydrophilic polymer brush surfaces with well-known high repellency against protein adsorption through surface-initiated atom transfer radical polymerization. Polymer brush layers have a well-defined surface structure: therefore, it could be a good model for clarifying the relationship between the surface structure and protein adsorption behavior. The influence of chemical structure of monomer unit and thickness of polymer brush layers on the adhesion force of proteins was discussed here, while that of graft density was not discussed. The adhesion force of bovine serum albumin (BSA) immobilized on an AFM cantilever against the thin polymer brush surfaces differed from the chemical structures of the monomer unit. The adhesion force of BSA decreased with increasing thickness of the polymer brush layer, and there was little difference in the adhesion force of BSA against the thick polymer brush surfaces regardless of the chemical structure of the monomer unit. The results demonstrate that the thickness of the polymer brush layer would be an important parameter that reduced the interaction between proteins and surfaces compared with the chemical structure of the monomer unit. (C) 2010 Elsevier Ltd. All rights reserved.