Whole human plasma contains a factor that inhibits the binding of bovine TSH to human thyroid membranes. To determine whether this activity is attributable to the presence of small amounts of immunoglobulin G (IgG) molecules that bind specifically to the thyroid, we have extracted from normal human plasma by a process of selective membrane adsorption a subfraction of IgG that is much more potent in TSH binding inhibition that the starting IgG. The enriched fraction was shown to be IgG by multiple criteria: precipitation in ammonium sulfate, elution by the anion exchange resin DEAE-cellulose, and electrophoresis in sodium dodecyl sulfate-urea polyacrylamide gel. Pretreatment with staphylococcal protein A, with specifically binds IgG, completely removed its activity. Significant TSH binding inhibition was retained under salt conditions, which have been shown to optimize the sensitivity and specificity of the TSH receptor. The enriched fraction was not an antimicrosomal or antithyroglobulin antibody, and did not bind to the TSH label. A similar enriched subfraction of bovine TSH binding inhibitory IgG could be prepared using membranes obtained from kidney and liver, suggesting that the membrane antigen with which it bound was not thyroid specific. These data indicate that in the plasma of individuals presumed to be free of thyroid disease there circulates low concentrations of an IgG which reacts with a thyroid membrane antigen(s). It may be an autoantibody or a normal constituent of plasma with specific binding properties.