We investigated four insulin-specific hybridoma antibodies with respect to their kinetic properties as well as the binding behaviour of some combinations of them. From equilibrium binding data all but one antibodies were shown to bear homogeneous binding sites. They revealed homogeneity of binding sites also by kinetic experiments, thus with high probability being monoclonal. At 0 degree C, two of them showed discrepancies between kinetic and steady state binding data in as much as, at steady state, the measured bound-to-free ratio of tracer insulin was 3-4 times lower than calculated from kinetic data. Thus a simple bimolecular reaction mechanism could possibly not be applicable. Mixing two monoclonal insulin antibodies, neither cooperative nor additive binding to the insulin molecule could be observed but only competitive effects. Especially, no positive cooperativity between two or more antibodies could be detected, which would be able to account for the higher affinity usually observed for polyclonal vs. monoclonal antibodies.