IFN-γ can either adversely or beneficially affect certain experimental autoimmune diseases. To study the role of IFN-γ in the autoantibody-mediated experimental autoimmune myasthenia gravis (EAMG), an animal model of myasthenia gravis in humans, IFN-γR-deficient (IFN-γR−/−) mutant C57BL/6 mice and congenic wild-type mice were immunized with Torpedo acetylcholine receptor (AChR) plus CFA. IFN-γR−/− mice exhibited significantly lower incidence and severity of muscle weakness, lower anti-AChR IgG Ab levels, and lower Ab affinity to AChR compared with wild-type mice. Passive transfer of serum from IFN-γR−/− mice induced less muscular weakness compared with serum from wild-type mice. In contrast, numbers of lymph node cells secreting IFN-γ and of those expressing IFN-γ mRNA were strongly augmented in the IFN-γR−/− mice, reflecting a failure of negative feedback circuits. Cytokine studies by in situ hybridization revealed lower levels of lymphoid cells expressing AChR-reactive IL-1β and TNF-α mRNA in AChR+ CFA-immunized IFN-γR−/− mice compared with wild-type mice. No differences were found for AChR-reactive cells expressing IL-4, IL-10, or TGF-β mRNA. These results indicate that IFN-γ promotes systemic humoral responses in EAMG by up-regulating the production and the affinity of anti-AChR autoantibodies, thereby contributing to susceptibility to EAMG in C57BL/6-type mice.