Avian erythrocytes possess insulin receptors which have binding properties that are virtually identical to those of the well studied mammalian insulin receptors. The affinity for porcine insulin was identical for the turkey and mammalian receptors over the entire range of insulin concentrations, as was the affinity of each of four insulin analogues which differed 300-fold in biological potency. Insulin induced acceleration of dissociation (i.e., the negatively cooperativite site-site interaction) was indistinguishable over a 10(6) range of insulin concentrations. Sharp pH dependence of binding was identical for turkey and mammalian receptors. The effects of temperature on association, dissociation and steady state binding were also identical. Thus, although birds and mammals have evolved separately for 300 million years there has been little change in the properties of the insulin receptor over this time period.