Following immunization with acetylcholine receptor (AChR), MHC class II-restricted, AChR-specific CD4 cell activation is critical for the development of experimental autoimmune myasthenia gravis (EAMG) in C57BL/6 mice. To study the contributions of B7-1 and B7-2 costimulatory molecules in EAMG, B7-1, B7-2, and B7-1/B7-2 gene knockout (KO) mice were immunized with Torpedo AChR in CFA. Compared with wild-type C57BL6 mice, B7-1 and B7-1/2 KO mice were resistant to EAMG development. B7-1 KO mice had reduced anti-AChR Ab compared with C57BL/6 mice. However, neither B7-1 nor B7-2 gene disruption impaired AChR-induced or dominant alpha(146-162) peptide-induced in vitro lymphoproliferative responses. Blocking of the B7-1 or B7-2 molecule by specific mAbs in vivo led to a reduction in the AChR-specific lymphocyte response, and the reduction was more pronounced in mice treated with anti-B7-2 Ab. The findings implicate B7-1 molecules as having a critical role in the induction of EAMG, and the resistance of B7-1 KO mice is associated with suppressed Immoral, rather than suppressed AChR-specific, T cell responses. The data also point to B7-2 molecules as being the dominant costimulatory molecules required for AChR-induced lymphocyte proliferation.