The mechanism underlying the bioinertness of nonfouling self-assembled monolayers was investigated with protein adsorption experiments, platelet adhesion tests, and surface force measurements with an atomic force microscope. Our force measurements revealed strong repulsion operating between nonfouling SAMs in the range of 4 to 6 nm in PBS buffer solution. In addition, we found that the protein-adsorbing and cell-adhering SAMs did not exhibit such repulsion. We concluded that the repulsion originated from structured interfacial water molecules. Considering the correlation among the above results, we propose that the structured interfacial water with a thickness of 2 to 3 nm (half of the range of the repulsion observed in the surface force measurements) plays an important role in deterring proteins and cells from adsorption or adhesion.