CD4+ T cells have been shown to be crucial in the development of experimental autoimmune myasthenia gravis (EAMG). The role of CD8+ T cells in EAMG is less well established. We previously showed that antibody depletion of CD8+ T cells in rats effectively suppresses EAMG. To further study the role and relationship of CD4+ versus CD8+ T cells in induction of EAMG, CD4-/-, CD8-/-, and CD4-8- mutant C57BL/6 mice and the parent CD4+8- wild-type mice were immunized with Torpedo acetylcholine receptor (AChR) plus complete Freund's adjuvant. Clinical EAMG was nearly completely prevented in CD4-8-, CD4-/-, and CD8-/- mice. This was associated with strongly reduced AChR-specific T and B cell responses, and with reduced levels of AChR-reactive interferon gamma (IFN-gamma) and interleukin 4 (IL-4) mRNA-expressing cells in lymphoid organs when compared with CD4+8+ wild-type mice. We conclude that (a) both CD4+ and CD8+ T cells are essential for development of EAMG, and a collaboration between these cell types may be necessary; (b) CD4+ as well as CD8+ T cells secrete IFN-gamma and IL-4, and both cytokines are involved in the development of EAMG; and (c), besides T cells, other immune cells might also be responsible for help of anti-AChR antibody production.