Using atomic force microscopy (AFM), we systematically studied the binding of three pairs of specific antigen/antibody systems : bovine serum albumin, tobacco etch virus capsid protein, and tobacco mosaic virus capsid protein and their respective specific antibodies. Our goals were to find a substrate for antigen immobilization, characterize individual antigen/antibody complexes, and investigate the antigen/antibody binding process. We found that the antigen protein can be immobilized on a -COOH-terminated self-assembled monolayer surface. Individual antigens and antigen/antibody complexes are easily identified from AFM images taken in liquid or under ambient laboratory conditions. The in situ studies suggest that antibody-antigen reactions occur in less than 4 min in buffer and that the reaction complexes are stable adsorbates once formed. As control experiments, nonspecific antibodies of equal and higher concentrations than those of specific antibodies have been used. There was no binding between nonspecific antibodies and the antigen-immobilized surfaces. The experimental results suggest that the procedure established here may be used for specific antibody detection. In addition, this study has also enhanced our understanding of antigen/antibody binding processes.