Using a strain of functioning rat thyroid cells (Fischer rat thyroid line-5, FRTL-5) as target cells and immunoglobin G sedimented from serum by polyethylene glycol (PEG), we studied thyroid stimulating antibody (TsAb) activity in serum of patients with Graves' disease. In a total of 92 cases, TsAb activity was detected in 92.59% (25/27) of the patients with untreated Graves' disease, 75.00% (9/12) of the patients with relapsing Graves' disease and 39.13% (18/46) of the patients with Graves' disease who were euthyroid while taking antithyroid drugs. TsAb activity was also detected in 42.86% (3/7) of the patients with ophthalmic Graves' disease and 12.50% (2/16) of the patients with Hashimoto's thyroiditis. The results showed that TsAb activity can be used as an immunologic index and is valuable in the diagnosis of Graves' disease. Remission and relapse of Graves' disease can be indicated or monitored with the measurement of TsAb activity.