Previous studies by us and others have shown that Graves' immunoglobulins G (IgGs) behaved as agonists or even antagonists of TSH. In this paper we have looked for the existence of IgG preparations without any thyroid stimulatory activity but able to significantly block the action of TSH in 128 hyperthyroid Graves' patients. The presence of TSH-binding inhibiting antibodies (TBIAb) and that of thyroid stimulating antibodies (TSAb) was evaluated by a radioreceptor assay using solubilized thyroid plasma membranes and by assaying the adenylate cyclase (AC) function of thyroid plasma membranes, respectively. Seventeen IgGs were negative for TSAb but positive for TBIAb in the screening, using only one concentration of IgG. Three kinds of activity were investigated in these IgGs at different doses: (1) TSH-binding inhibiting activity; (2) thyroid AC stimulating activity; and (3) the inhibition of TSH-induced AC stimulation. The results showed that the level of activity was not always dose-dependent. A significant (greater than 20%) inhibition of the TSH-dependent AC stimulation was present in 15 of the 17 IgGs examined: this inhibition was more elevated at lower than at higher doses in two preparations. No significant correlation was found between the three activities. In short, we have been able to demonstrate the existence of 'blocking' antibodies, apparently without any stimulatory activity, in some patients with Graves' disease. The diphasic pattern of the dose-response curves of some IgGs and the lack of correlation between the different activities can be explained by the co-existence in the sera of Graves' patients of different autoantibodies varying in concentration, binding affinity constant and intrinsic biological activity.