Monoclonal antibodies which bind small drugs are useful for the study of the interactive forces involved in antibody-ligand complexation. Detailed understanding of these supramolecular forces requires a careful examination of structural and thermodynamic parameters of the interacting molecules. Fluorescence spectroscopy techniques are very useful in this regard. We report here, the kinetic and energetic parameters of four monoclonal antibodies made against the tricyclic antidepressant imipramine. These monoclonal antibodies were found to possess high to very high binding affinity constants, ranging from 10(7) to 10(10) M-1, and caused fluorescence quenching or enhancement of a fluorescein labelled imipramine. The dissociation rates of the fluorescent ligand from the complexes were measured at different temperatures in order to provide some insight regarding the kinetic and energetic (thermodynamic) parameters of the antibody-ligand binding interactions.