T hyrotropin receptor (TSHR) Ab’s of the stimulating variety are the cause of hyperthyroid Graves disease. MS-1 is a hamster mAb with TSHR-stimulating activity. To examine the in vivo biological activity of MS-1, mice were treated with purified MS-1 intraperitoneally and the thyroid response evaluated. MS-1 induced a dose-dependent increase in serum thyroxine (T4), with a maximum effect after 10 ∝g of MS-1 was administered. MS-1–secreting hybridoma cells were then transferred into the peritoneum of nude mice to study chronic thyroid stimulation. Serum MS-1 levels detected after 2 weeks were approximately 10–50 ∝g/ml, and the serum TSH was suppressed in all animals. Serum triiodothyronine levels were elevated, but only in animals with low serum MS-1 concentrations. In addition, there was a negative correlation between serum T4 and the serum MS-1 concentrations. These in vivo studies suggested a partial TSHR inactivation induced by excessive stimulation by MS-1. We confirmed this inactivation by demonstrating MS-1 modulation of TSHR function in vitro as evidenced by downregulation and desensitization of the TSHR at concentrations of MS-1 achieved in the in vivo studies. Thus, inactivation of the TSHR by stimulating TSHR autoantibodies (TSHR-Ab’s) in Graves disease patients may provide a functional explanation for the poor correlation between thyroid function and serum TSHR-Ab concentrations.