Thyrotropin binding inhibitory immunoglobulin (TBII) and thyroid stimulating antibody (TSAb) activities were measured in the thyroidal and peripheral venous blood samples at the time of subtotal thyroidectomy from twenty one patients with Graves' disease prepared for surgery with antithyroid drugs. There was no difference in TBII and TSAb activities between thyroidal and peripheral blood samples. These findings were regarded that while intrathyroidal lymphocytes are major site of thyrotropin receptor antibody (TRAb) production, similar levels are found in thyroidal and peripheral veins and that this in vive study cannot exactly ascertain the TRAb producing site.