Sera from 38 myasthenic patients and 15 non-myasthenic controls were surveyed by indirect immunofluorescence technique for reactivity against skeletal muscle, nuclei and mitochondria. Cell-mediated response to purified muscle antigen was detected by leukocyte migration inhibition assay. The positive rates for antiskeletal antibody (ASMA), antinuclear antibody (ANA), and antimitochondrial antibody (AMA) were 50%, 10.5% and 0%, respectively, in the myasthenic patients, and were 13.3%, 6.6% and 0%, respectively, in the controls. The mean titer of antiskeletal muscle antibody was 1:9.8 in the myasthenic patients and 1:1.8 in the controls. Both the positive rate and the mean titer of ASMA were higher and the migration index was lower in the patients with thymoma or thymic hyperplasia. To the contrary, among the group treated with corticosteroids, the correlation of the positive rate and mean titer of ASMA to migration index revealed a marginally significantly negative linear correlation (y = -0.72 chi + 84.69, r = -0.80, 0.1 greater than p greater than 0.05). This meant that the higher positive rate of ASMA was usually associated with the lower migration index. The facts of higher positive rate of ASMA in myasthenic patients and its negative correlation to migration index gave supportive evidence for a possible role of ASMA in the pathogenesis of myasthenia gravis.