Administration of sera from patients with active Graves' disease to nude mice bearing human thyroid xenografts provides the opportunity to compare directly the effects of TSAb on thyroid tissue of different species. The results presented in this paper, namely the fact that only one of 8 Graves' sera increased the RAIU of the nu/nu mouse thyroids modestly while all sera increased the RAIU in the transplanted human tissue considerable, clearly show a species specific effect of TSAb.