The thyrotropin receptor from bovine thyroid plasma membranes has been solubilized using lithium diiodosalicylate, and an assay to measure thyrotropin binding to the solubilized receptor has been developed. Both the solubilized thyrotropin receptor and the thyrotropin receptor on thyroid plasma membranes have effectively identical nonlinear Scatchard plots and negatively sloped Hill plots, i.e. both preparations have receptors which appear to exhibit a similar negatively cooperative relationship. Although the pH optimum of thyrotropin binding to the solubilized receptor is the same as that of the thyroid plasma membrane receptor, pH 6.0, the pH dependency curve of the solubilized receptor is slightly different in its outline. Thyrotropin binding to the solubilized receptor is less sensitive to salt inhibition than is binding to the thyroid plasma membrane receptor; however, optimal binding remains at 0 degrees. The relative affinities of thyrotropin and two glycoprotein hormones which can be considered structural analogs, luteinizing hormone and human chorionic gonadotropin, are 100:10:5, respectively, toward plasma membrane receptors, but 100:25:40 toward the solubilized receptors. The solubilized receptor preparation is heterogeneous in size in that it has binding components with molecular weights of 286,000, 160,000, 75,000, and 15,000 to 30,000. Tryptic digestion converts all three higher molecular weight components to the 15,000 to 30,000 molecular weight species, and the 15,000 to 30,000 molecular weight receptor component has all of the binding properties of the solubilized receptor preparation before tryptic digestion including an identical nonlinear Scatchard plot. It has the same size as and coelutes from Sephadex G-100 with a 15,000 to 30,000 molecular weight receptor released by tryptic digestion of bovine thyroid plasma membranes or tryptic digestion of bovine or dog thyroid cells in culture. The tryptic fragment of the solubilized receptor or preparations has been purified almost 250-fold by affinity chromatography on thyrotropin-Sepharose columns. The binding activity is lost when the solubilized thyrotropin receptor preparation is exposed to beads of neuraminidase-Sepharose or conconavalin A-Sepharose.