Photostimulated association and dissociation of anti-dinitrophenyl antibody, anti-DNP-Ab, to and from a dinitrospiropyran photoisomerizable monolayer is probed by surface plasmon resonance (SPR). The anti-DNP-Ab binds to a dinitrospiropyran antigen monolayer, whereas the DNP-Ab dissociates from the protonated dinitromerocyanine monolayer. Reversible binding and dissociation of anti-DNP-Ab to and from the photoisomerizable monolayer was assayed by SPR spectroscopy. Analysis of the SPR data indicates that the average thickness of the antibody layer is ca. 70 Å which corresponds to a surface coverage of ca. 4.4 × 10-9 mol·cm-2.