Bovine serum albumin and γ-globulin were adsorbed, respectively, on films of ABA-type block copolymers and blendmers composed of 2-hydroxyethyl methacrylate (HEMA) and styrene in a mixture of protein and 0.2 M of phosphate buffer solution (pH 7.4) under the condition of 1/50 physiological concentration of the protein at 37°C. The morphologies and dimensions of the microphase separated structure of the block copolymers and blendmers containing hydrophilic domains of HEMA molecules and hydrophobic domains of styrene molecules were studied by electron microscopy with or without osmium tetroxide fixation. In the adsorption of plasma protein, either both the adsorbed plasma protein and the hydrophilic domains or only the adsorbed plasma proteins could be stained selectively with osmium tetroxide to control the fixation time. Therefore the areas of plasma protein adsorbed on the phase of microseparated structure were evaluated by comparing the micrographs with and without the adsorbed plasma proteins. These results show that bovine serum albumin is adsorbed on the hydrophilic domains escaping from the hydrophobic domains, and on the contrary that γ-globulin is selectively adsorbed on the hydrophobic domains.