In-vitro mouse thyroglobulin (MTg) activated spleen cells from immunized donor mice can induce experimental autoimmune thyroiditis (EAT) after transfer to recipient mice. The intrathyroidal cellular infiltrate consists primarily of mononuclear cells (lymphocytic EAT). Cells cultured with MTg together with anti-IL2R antibody induce EAT with a granulomatous hisopathology in which the thyroid infiltrate contains mononuclear cells (MNC) in addition to PMN's, histiocytes, and multinucleated giant cells. Flow cytometric analysis of intrathyroidal MNC infiltrates demonstrated that both CD4 + and CD8 + T cells infiltrate the thyroid in both lymphocytic and granulomatous EAT and that CD8 + T cells outnumber CD4 + T cells. There were usually increased numbers of PMN's in the granulomatous thyroids, but low number of Ig + and F4/80 + cells (macrophages) in the intrathyroidal infiltrate of both disease types. IL2R and Pgp-1 were expressed on both CD4+ and CD8+ intrathyroidal T cells. The majority of CD8 + cells were 1CAM +, LFA-1 +, and CD45RB + whereas only a small percentage of CD4 + intrathyroidal T cells expressed these markers. There were no major differences in intrathyroidal MNC phenotype between lymphocytic and granulomatous EAT. Depletion of CD8 + T cells in recipient mice did not reduce EAT severity and resulted in an increased percentage of intrathyroidal CD4 + T cells expressing 1L2R. These results suggest that CD8 + T cells are not functioning as effector cells in lymphocytic or granulomatous EAT.