The dissociation of antibodies bound to surface-immobilized antigen was investigated by the ELISA, using a hapten (TNP) as antigen. Antibody binding was found to be stable, and no half-time of dissociation could be defined within 69 h. The role of the bivalence of antibodies and the difference between a homogeneous and a heterogeneous reaction was investigated by comparing the dissociation rate of antigen-antibody complexes formed by monovalent Fab fragments from surface-immobilized antigen and the dissociation rate of TNP-lysine from antibodies in a homogeneous liquid phase. Fab fragments were found to dissociate with a half-time value of about 16 h, whereas the homogeneous binding of TNP-antibody dissociated with a half-time of less than 4 h, indicating that both the bivalence of antibodies and the solid phase contributed to the stability of surface-bound antigen-antibody complexes. Qualitative differences between antibodies produced by different clones in a polyclonal antibody response to TNP was investigated by a spot assay. The results indicated that a minority of the antibodies produced had the capacity of binding practically irreversibly to solid-phase-immobilized antigen. The impact of the results on the interpretation of data from solid-phase assays is discussed together with the biological importance of the findings.