Immunoglobulin G purified from the serum of patients with Graves' disease, stimulated the thyroid of man, calf, and guinea pig (cAMP accumulation as the end-point) in vitro, and the thyroid of the mouse in vivo (LATS bioassay). The stimulatory effect on the thyroids of all four species was removed by adsorption of the immunoglobulin G to human thyroid membranes and was diminished to a proportionately similar degree by adsorption with bovine thyroid membranes. All activity, as assessed by stimulation of the human thyroid in vitro. was recovered from both human and bovine membranes by elution with 2 M NaSCN solution. The data support the concept that the thyroid-stimulating antibody of Graves' disease is homologous to a human thyroid antigen and, in some instances, cross-reacts with a similar antigen in the thyroid of a distant species and so stimulates the heterologous gland.