The IGF-I receptor binds IGF-I with complex kinetics characterized by a curvilinear Scatchard plot, suggesting receptor heterogeneity and apparent negative cooperativity. To explore the molecular mechanisms underlying these properties, we have characterized the binding of a hybrid receptor formed from a wild-type receptor monomer and a mutant receptor monomer devoid of binding activity. Receptor hybrids were generated by transient co-transfection of cDNAs encoding wild-type and Mutant receptors with unique epitope tags. Hybrid receptors were purified front transfected cells by sequential immuno-affinity chromatography and their ligand-binding properties were determined. Complementation produced it hybrid with near wild-type affinity. Dissociation studies demonstrated that the hybrid did not exhibit negative cooperativity. © 2005 Elsevier Inc. All rights reserved.