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The kinetics of insulin binding to its receptors on human erythrocytes suggests the existence of two types of receptors exhibiting negative cooperativity upon the binding of the hormone. Solubilized and purified receptors were associated to Zn++ or Cu++. The addition of these ions to erythrocytes or to purified insulin receptors from human erythrocytes resulted in an increase of specific insulin binding. Dialysis of solubilized or purified receptors against chelating agents such as EDTA or 1, 10-phenanthroline resulted in a decrease in specific binding of insulin. With the readdition of Zn++ or Cu++ to the medium an increase in specific binding was observed, and values much higher than those of the original preparations were obtained. Dialysis of purified receptors against chelating agents resulted in a decrease in the content of Zn++ and Cu++. These results suggest the possible involvement of a metal ion associated to the receptor in the formation of the insulin-receptor complex.