The IgG subclass distribution of non-receptor muscle antibodies was examined in 15 myasthenia gravis (MG) sera, employing an indirect haemagglutination-immunofluorescence technique. Four sera contained only IgGl, 4 contained only IgG4 and 7 contained both IgGl and IgG4 muscle antibodies. IgG2 and IgG3 antibodies were not found. Among 11 patients with a defined thymus pathology 8 had thymoma and 3 had thymic atrophy, but there was no correlation between antibody subclass pattern and thymic pathology. Patients with both IgG1 and IgG4 antibodies tended to have the longest disease duration. We conclude that IgG non-receptor muscle antibodies in MG are of the IgG1 and/or IgG4 subclasses, irrespective of thymic pathology.