One distinguishing feature of "life" is that the physical forces between biological molecules and membrane surfaces are often highly specific, in contrast to nonspecific interactions such as van der Waals, hydrophobic, and electrostatic (Coulombic) forces. We have used the surface-forces-apparatus technique to study the specific "lock and key" or "ligand-receptor" interaction between two model biomembrane surfaces in aqueous solution. The membranes were lipid bilayers supported on mica surfaces; one carrying streptavidin receptors, the other exposing biotin ligand groups. We found that, although no unusual or specific interaction occurs between two avidin or two biotin surfaces, an avidin and a biotin surface exhibit a very strong, very short-range (less than 1 nm) attraction and that the binding mechanism involves equally specific molecular rearrangements. The results also show that highly specific biological interactions such as are involved in immunological recognition and cell-cell contacts may be studied at the molecular level and in real time by the surface-forces-apparatus technique.