Antifilamin, antivinculin, and antitropomyosin antibody activities were investigated in sera from 43 patients with myasthenia gravis (MG). Antifilamin and antivinculin antibody activities are significantly higher in MG patients compared with normal controls and patients with Duchenne muscular dystrophy (DMD), Parkinson's disease, and spinocerebellar degeneration. The antifilamin antibody was highly positive (100%) in ocular myasthenia. Antitropomyosin antibody activity was similar to that in DMD patients. No correlation was observed between these antibody activities and the antiacetylcholine receptor antibody titers, the duration of the disease, and thymic pathology.