We investigated the interaction between acetylcholine receptor (AChR)-specific T-helper cells from patients with myasthenia gravis and murine monoclonal anti-AChR antibodies. At optimal antigen concentration, anti-AChR antibodies neither enhanced nor impaired T-cell responses. However, at substimulatory antigen concentration, addition of anti-AChR antibodies substantially enhanced the proliferation of AChR-specific T cells. In spite of low amounts of antigen, immune complex formation allowed highly efficient capture and uptake of antigen via Fc receptors on antigen-presenting cells, which could be inhibited by an antibody to Fc receptors. Immune complex-mediated stimulation of sensitized AChR-specific T lymphocytes in vivo may contribute to the exacerbation of the disease, and demonstrates the interaction between T and B lymphocytes in myasthenia gravis.