Serum complements of C1q C1, C4, BF, C1-INH and C5 were measured by single immunodiffusion in 54 patients with myasthenia gravis (MG), 46 normal control (NC) and 42 cases of other neurological disorders. It was found that C1-INH, C5 levels in MG were significantly higher than that in other groups (P less than 0.01). The serum levels of C1-INH, C5 were not related to the clinical type and the stage of the illness. These data suggest that endplate receptors might be disrupted through complement mediated immunoreaction.