Analysis of the course of D-glucose binding to insulin has shown that the mechanism of glucose-insulin interaction is a cooperative process. Binding of D-glucose molecules to insulin is facilitated by the dissociation of insulin aggregates caused by insulin-glucose interaction. Thus, insulin behaves as a system with strong positive cooperativity. The results have been treated in accordance with theories for interactions coupled to association equilibria. The data obtained support the idea that insulin monomers are the active species responsible for insulin action.