The damage of acetylcholine receptor (AChR) at neuromuscular junctions of experimental autoimmune myasthenia gravis (EAMG), an animal model of human MG, is mediated by B cells which require T cell help. The Th2 associated cytokine IL-10 suppresses production of cytokines released by Th1 cells and is considered for treatment of human autoimmune diseases. To evaluate the role of IL-10 in EAMG, rhIL-10 was administered daily to Lewis rats by the subcutaneous route starting at the day of immunization and continued for 7 weeks. IL-10 failed to abrogate EAMG at low dose (0.1 or 1 microg/day) and at the dose of 3 microg/day caused earlier onset and aggravated clinical signs of EAMG when compared to EAMG rats injected with PBS only. Although Th1 responses reflected by AChR-induced lymphocyte proliferation and levels of IFN-gamma secreting cells, as well as AChR-induced Th1 cytokine mRNA expression was suppressed, augmented IL-4 mRNA expression and AChR-specific B cell responses may play an important role in the failure of IL-10 to abrogate EAMG. This study implicates a critical precaution in planning immunotherapy of IL-10 in antibody-mediated autoimmune diseases, e.g. MG.