The interaction of insulin with a purified and well characterized preparation of liver plasma membranes has been studied. The time course of binding of ¹²⁵I-insulin and displacement of tracer at the plateau of binding was measured as a function of temperature and ionic strength. Hormone tracer concentrations were in the low physiologic range. The reaction is complex and includes reversible binding of insulin to its receptor and degradation of both insulin and receptor. The degrading reactions were minimized by using low to medium membrane concentrations and were corrected for by measuring the intact insulin remaining. When the binding reaction was studied over the range of 0.06 to 1000 ng per ml of insulin at 30°, the data fit a model with a minimum of three classes of receptor sites: a high affinity low capacity site with a K of 2.0 x 10⁹m^-1 and a capacity of 5 x 10^-14 moles per mg of membrane protein; a low affinity-high capacity site with a K of 2.1 x 10⁸m^-1 and a capacity of 1.5 x 10^-12 moles per mg of membrane protein; and a very low affinity-high capacity site (“nonspecific site”) representing about 5% of the total tracer binding. Both the affinity and binding capacity are influenced by temperature and ionic strength. Kinetic data for insulin receptor complex dissociation are also consistent with heterogeneity of receptor sites. Discrepancies in the literature between the number of reported classes of insulin receptor sites and their affinity constants are discussed in terms of the variables of this complex reaction.