Plasma membranes have been purified from porcine thyroid gland homogenate by discontinuous sucrose gradient centrifugation. The preparations contained specific binding sites for thyrotropin but not for luteinizing hormone or the β subunits of thyrotropin and luteinizing hormone. Optimum conditions of 125I-labeled thyrotropin binding were pH 6.0–6.5 and 37°C. Thyrotropin binding was reduced by divalent (Ca2+, Mg2+) and monovalent cations (Na+, K+, Li+), 50% inhibition being obtained at 10 mM and 50 mM respectively. Displacement curves of 125I-labeled bovine or porcine thyrotropin by the unlabeled hormone from three species was in the order of increasing concentrations (bovine > porcine > human) which is the order of decreasing biological activity of these hormone preparations in the assay in vivo in the mouse. The validity of the results was established by controlling that porcine membranes bound the native and the 125I-labeled hormones with equal affinity. A single type of high-affinity (Kd= 0.28 nM) binding sites was detected for bovine and porcine thyrotropins. In contrast, porcine plasma membranes bound human thyrotropin with a lower affinity (Kd= 70 nM). A good correlation was found at equilibrium and in the conditions of the cyclase assay, between receptor occupancy and adenylate cyclase activation for the three hormones.