The characteristics of binding of 125I-labeled bovine thyrotropin to bovine thyroid plasma membranes were studied. At pH 7.5, 0 C, specific binding was maximal in 3 hr and was progressively inhibited by Na+, Mg++, Ca++ and (NH4)2 SO4 in concentrations of 25 mM and greater. Affinity constants of l.047 X 10(8)M-1 and 0.57 X 10(6)M-1 were obtained for two binding sites and Hill plots provided a mean slope of 0.86, suggesting negative cooperativity. Addition of 1-thyroxine or 1-triiodothyronine at 0.5 ng to 50 microgram per ml had no significant effect but propylthiouracil (10(-7) to 10(-5)M) and KC104 (10(-7) to 10(-4)M) had biphasic effects, first enhancing and then inhibiting binding. Normal human serum IgG, particularly two fractions (3 and 4) obtained by chromatography on columns of cellulose phosphate, and an alpha-globulin - rich fraction from chromatography of serum proteins on diethylaminoethyl cellulose, inhibited binding at concentrations of 0.5 mg per ml or greater. Thus two possibilities are that normal human serum globulins either compete with thyrotropin for its receptor on thyroid plasma membranes or, in binding to the membranes, cause such perturbation as to affect adversely the conformation of the receptor for thyrotropin.