Thyroid-stimulating antibodies were determined with cyclic AMP accumulation induced by unfractionaled serum using cultured human thyroid cells. Thyroid-stimulating antibodies were found in 83.3% (25/30) of patients with Graves' disease without treatment, 43.8% (7/16) of patients with Graves' disease during antithyroid treatment, 4% (1/25) of normal people and none of patients with simple goitre or thyroid adenoma. The results indicate that determination of thyroid-stimulating antibodies using this method is sensitive and characteristic in patients with Graves' disease.