In the present study, we attempted to define possible subpopulations of antibodies which theoretically could be directed against evolutionarily conserved regions of the insulin molecule in sera from insulin-treated diabetic patients using a variety of labelled and unlabelled insulins which differ widely in structure but are very similar in functional properties. Ten high titre human insulin antisera from patients treated with mixed beef-pork insulin were examined. All sera were found to bind 125I-pork insulin better than labelled chicken insulin which bound better than labelled fish insulin. Detailed studies were conducted with four of the antisera using the pork and fish tracers. With two of the antisera, a subpopulation of antibody could be detected with 125I-fish insulin which had similar affinity for both fish and pork insulin, but reacted much less well with guinea pig insulin and the desoctapeptide derivative of porcine insulin. Based on the known properties of these four insulins, the data provide suggestive evidence consistent with the hypothesis that there are subpopulations of antibodies recognizing regions on the insulin molecule that are well conserved, possibly the region involved in the formation of insulin dimers or receptor binding.