We have analyzed the human T-cell receptor (hTcR) Va gene repertoire in thyroid tissue transplants of a patient with hyperthyroid Graves' disease. Blocks of thyroid tissue were transplanted subcutaneously into 10 mice with severe immunodeficiency (scid) and 4 weeks later 5 of the mice were injected intraperitoneally with autologous peripheral blood mononuclear cells (PBMC) (107cells per mouse). After a further 3 weeks, mice were sacrificed and total cellular RNA and cDNA prepared from each of the explants. We used specific olingonucleotides in polymerase chain reactions (PCR) to amplify 18 different human hTcR Va gene families and the identity of the PCR fragments was confirmed by Southern blot analysis.Different samples of the donor thyroid tissue consistently expressed 9–10 of the 18 hTcR Vαgene families screened (Vα1–7, 11, 12&15). A more marked bias in hTcR V gene family use was seen in each of the explants with a mean of only 2.8 Vαgene families detected. After 7 weeks of transplantation, the thyroid explants largely reflected some of the same genes seen in the hTcR V gene repertoire of the donor tissue with particularly pronounced expression of Vα2 and Vα3 gene families. The transplantation of PBMC into thescidmice showed evidence for their accumulation within the transplanted thyroid tissues as judged by the appearence of additional hTcR V gene families expressed in these samples although the specificity of such accumulation remains unclear. PCR fragment sequencing showed evidence that the hTcR Vα3 PCR products represented a mixture of clonaliy explanded and heterogeneous T cells while a randomly selected Vα13 product showed only evidence of heterogeneous T cells. Hence, at least the Vα3 gene family is likely to be involved in the etiopathogenesis of autoimmune thyroid disease in this particular patient.Our data underline the value of thescidmouse as anin vivomodel to study the human intrathyroidal lymphocyte population in Graves' disease and confirm the presence of selected T cells within the thyroid gland of patients with Graves' disease.