Macrophage-cytophilic antibodies, with acetylcholine receptor specificity, are present in rabbits with experimental autoimmune myasthenia gravis (EAMG). Such antibodies may have a significant role in the immunologic mechanism involved in the pathogenesis of myasthenia gravis. The induction of EAMG in rabbits was performed by injection of purified AChR from the electric organ of Torpedo californica. Both the binding of AChR specific cytophilic antibodies to normal macrophages and the presence of such antibodies bound in vivo to macrophages of sick animals were demonstrated in vitro. The amount of cytophilic antibodies was determined by measuring cell-associated radioactivity after the addition of 125I-AChR. Cytophilic antibodies capable of binding to normal alveolar macrophages were detected in all animals 14 days after immunization, and were maintained through the severe stages of the disease. In addition, cytophilic anti-AChR antibodies were shown to be bound in vivo to alveolar macrophages drawn from severely sick rabbits, as was measured by a direct 125I-AChR binding to such macrophages. The role of cytophilic antibodies and in particular of macrophage-associated cytophilic antibodies in the pathogenesis of autoimmune diseases is not clear yet. The availability of AChR specific cytophilic antibodies can now be utilized for studying their role in the immunopathogenesis of EAMG.