Molecular recognition forms the basis for assembly and regulation in living organisms. We have used the atomic force microscope (AFM) to study the interaction of a model receptor streptavidin with its ligand biotin under physiological conditions. Surfaces functionalized with biotin and streptavidin exhibited adhesive forces 3-8 times greater than the nonspecific interactions observed between blocked streptavidin and biotinylated surfaces. The magnitude and distribution of the observed adhesive forces suggest they result from individual streptavidin-biotin interactions. This technique provides a means to directly study molecular recognition interactions at the molecular level.