Estradiol-displayed bioaffinity beads binding to the anti-estradiol antibody attached via the protein A-coated mica surface were examined by atomic force microscopy (AFM). The amount of specifically bound beads on the surface was directly proportional to the concentration of free estradiol in solution. Estradiol from 10 ng ml(-1) to 10 mug ml(-1) could be determined. This suggested that direct counting of bioaffnity beads by AFM can be used to detect specific ligand for the target protein.