The presence of antibodies to skeletal muscle in the sera of some patients with myasthenia gravis was first demonstrated by direct and complement immunofluorescent staining by Strauss et al.1 Subsequently, we found that these antibodies may be demonstrated by the indirect staining method as well, that they combine also with heart muscle (thus they are referred to as SH antibodies), and that they behave as autoantibodies,2 ie, the patient's serum reacted with his own muscle tissue in the in vitro test system. A second antibody which is less commonly found in sera of patients with myasthenia gravis reacts only with skeletal muscle. This so-called S antibody was also demonstrated to be an autoantibody by a test-tube complement-fixation-test method. More recently a highly sensitive tanned-cell agglutination test for antibodies to skeletal and heart muscle has been elaborated in this laboratory.3 Again the SH antibodies were found to combine