Prevention of protein adsorption by the surface-grafted poly(ethylene oxide) (PEG) chains has been well-known. We have examined the mechanisms of how the grafted PEO prevents protein adsorption. PEO-poly(propylene oxide)-PEO (PEO-PPO-PEO) triblock copolymers were used to graft PEO to the trichlorovinylsilane (TCVS)-modified glass by gamma-irradiation. The surface density of the PEO chains was varied up to 60 pmol/cm(2) and the number of the ethylene oxide (EG) units of the PEG segment was varied from 75 to 128. The adsorption of lysozyme and fibrinogen to the PEG-grafted glass was examined using radiolabeled proteins. The surface protein concentration decreased as the surface density of the grafted PEO increased, but surface protein concentration never reached zero. The experimental data. were compared with the predictions by the single-chain mean-field theory. There was very good agreement between the predictions of the theory and the experimental observations. It was found that the mechanism for prevention of protein adsorption by the grafted PEO chains in the hydrophobic surfaces was due to the blocking by the PEO segments of the adsorbing sites of the proteins. The mechanism of the grafted chains to prevent protein adsorption was shown to depend upon the interactions of the surface with the segments of the grafted polymers. Surfaces that did not attract the polymer segments present effective kinetic barriers but were not very good for equilibrium prevention. On the other hand, hydrophobic surfaces, such as the ones used in the experimental work, were very effective for reducing the equilibrium amount of proteins adsorbed. It was found that the most important parameter in preventing protein adsorption by grafted polymers is the surface density of the grafted polymer. The polymer molecular weight, or the chain length, was found to have a weak. effect.