To investigate T-cell sensitization to thyroid autoantigens in autoimmune thyroid disease (AITD), purified soluble human thyroglobulin (Tg) and thyroid peroxidase (TPO) were used. Peripheral blood mononuclear cells (PBMC) as well as CD8-depleted, CD4-enriched PBMC (“selected”PBMC) from 9 patients with Graves' disease (GD). 13 Hashimoto's thyroiditis (HT) and 10 healthy subjects, were cultured for 6 days with or without varying concentrations (0.1, 1.0 and 5.0μg/ml. respectively) of Tg or TPO and their responses were evaluated using the3H-thymidine incorporation assay.Total PBMC as well as selected PBMC from GD and HT responded to both TPO and Tg, but normal PBMC did not. This induction was more marked in selected PBMC; on the other hand, CD8 depletion did not permit normal PBMC to respond to either antigen. However, reactivity of selected AITD PBMC to Tg differed from that of TPO. Two way analysis of variance showed that the proliferative response was significantly greater with Tg than with TPO, (again particularly notable with the“selected”PBMC) in both GD and HT. There was no difference between control and AITD preparations when an irrelevant (renal microsomal) antigen was employed. Taken together with our previous report that CD4 cells were induced by TPO even when cultured with CD8 cells, it is evident that suppressor CD8 cells do play a role in CD4 cells from proliferating against Tg and TPO however their function alone or in combination with suppressor-inducer CD4 cells is partially disturbed. so that T cell sensitization to Tg and TPO can be identified in the AITD PBMC.