Molecular recognition between biotinylated bovine serum albumin and polyclonal, biotin-directed IgG antibodies has been measured directly under various buffer conditions using an atomic force microscope (AFM). It was found that even highly structured molecules such as IgG antibodies preserve their specific affinity to their antigens when probed with an AFM in the force mode. Evidence that only a very limited number (less than 10) antibody/antigen pairs contribute to the measured interaction force is given. The potential and limitations of this new approach for the measurement of individual antigen/antibody interactions, and some possible applications are discussed.