The force acting between a protein molecule and a nonbiological surface is of great importance in biotechnology. We adsorb protein-covered microspheres on glass and polystyrene substrates in Tris-buffered saline. Using a novel atomic force microscopy technique, we show that proteins adsorb better on hydrophobic than hydrophilic surfaces and that the microspheres adhere with a force characteristic of the particular protein and substrate. The adhesion force on the hydrophobic polystyrene substrate is shown to depend on the structural rigidity of the protein, while on the hydrophilic glass surface, protein and surface charge is more important.