Anti TSH-receptor antibodies (TBIAb) were measured by a radioreceptor assay in 277 patients with Graves' disease, 101 with autoimmune thyroiditis, 43 with autonomous adenoma, 15 with subacute thyroiditis, 15 with euthyroid ophthalmopathy, 155 with euthyroid multinodular goiter, 10 with amiodarone-induced hyperthyroidism and 2 with tumoral TSH hypersecretion. TBIAb were present at high titers in 74% of patients with untreated or relapsed Graves' disease and, at lower titers, in only 10% of patients who had recovered from Graves' disease, in 8% of patients with autoimmune thyroiditis and in 4% of patients with euthyroid goiter. TBIAb were absent in normal subjects as well as in the other groups studied. These findings suggest that TBIAb represent a specific marker of Graves' disease, particularly of the untreated form. Their presence in non Graves' patients may be considered expression of inactive or inhibiting antibodies.