Spleen cells from CBA/J mice immunized with mouse thyroglobulin (MTg) and the adjuvant lipopolysaccharide induce experimental autoimmune thyroiditis (EAT) after transfer to recipient mice if they are first activated in vitro with MTg. EAT induced by cells cultured with MTg is generally moderate in severity and is characterized by a thyroid infiltration consisting primarily of mononuclear cells. Addition of the anti-interleukin 2 receptor (IL-2R) monoclonal antibodies (mAbs) M7/20, 3C7, or 7D4 to spleen cell cultures with MTg resulted in a cell population capable of inducing a more severe type of EAT characterized by extensive follicular destruction, granuloma formation, and the presence of multinucleated giant cells. Recipients of cells cultured with MTg and anti-IL-2R mAb also had higher anti-MTg autoantibody responses than recipients of cells cultured with MTg alone. Activation of cells capable of transferring severe granulomatous EAT and increased anti-MTg autoantibody responses required both MTg and M7/20 in culture and required addition of M7/20 within the first 8 h of the 72-h culture period. CD4+ T cells were required for the expression of both the severe granulomatous EAT lesions and the mononuclear cell infiltrates typically observed in murine EAT. The increased anti-MTg autoantibody responses in recipients of cells cultured with MTg and anti-IL-2R mAbs were not restricted to a particular immunoglobulin G (IgG) subclass and included antibody of the IgG1, IgG2A, and IgG2B subclasses. These results suggest that a subset of CD4+ T cells capable of inducing severe granulomatous EAT and increased anti-MTg autoantibody responses is preferentially activated when cells are cultured in the presence of anti-IL-2R mAb. Anti-IL-2R mAb may either prevent activation of cells that induce classical lymphocytic EAT or prevent activation of cells that normally function to downregulate EAT effector T cell activity.