Susceptibility to experimental autoimrnune thyroiditis (EAT) in mice is linked to the I-A subregion of the major histocompatibility complex (MHC). The present study was undertaken to assess the effectiveness of anti-I-Akmonoclonal antibody (MAb) 10–2.16 in preventing or arresting the development of EAT. Spleen cells from CBA/J or (CBA/J x Balb/c) Fl mice given 10–2.16 prior to sensitization with mouse thyro-globulin (MTg) and adjuvant could not transfer EAT to normal recipients, and cells from these mice did not proliferatein vitroto MTg. Donor CBA/J mice given 10–2.16 before immunization and recipients of cells from such mice produced little MTg-specific lgGl or IgG2b antibody but did produce nearly as much IgG2a as controls. The effects ofin vivotreatment with 10–2. 16 appear to be due to elimination of Ia + cells rather than to modulation of Ia or induction of suppressor T cells. When 10–2.16 was added toin vitrocultures it also prevented the proliferation and activation of sensitized CBA/J or Fl effector cell precursors. Other mAb specific for MHC class II gene products, but not associated with disease susceptibility, expressed by CBA/J (I-Ek) or FI (I-Ad) mice (14-4-4S or MK-D6 respectively), also preventedin vivosensitization, but did not blockin vitroactivation. Anti-I-Akwas also effective in preventing EAT if multiple injections of mAb were given to recipients of sensitized EAT effector cells. These studies indicate that Ia + cells are required for initial sensitization and activation of EAT effector cells and also contribute to the development of severe histopathologic lesions in the thyroid during the final effector stage of EAT.