High-affinity IL-2-R binding results from an exceptional type of cooperative interaction between two IL-2-binding proteins termed alpha and beta. When expressed together on the cell surface, these two distinct chains form a noncovalent kinetic hybrid receptor complex that exploits a rapid association rate contributed by the p55 beta chain and a slow dissociation rate characteristic for the p75 alpha chain. The p75 alpha chains signal cell growth, whereas the p55 beta chains only facilitate IL-2 binding by serving as helper binding sites, having no discernible signaling role themselves. The unique functional implications of this structural organization indicate that this cooperative bimolecular arrangement reflects a general mechanism by which the efficiency of surface receptors can be enhanced markedly.