Citric acid extracts (CAE) of human skeletal muscle were prepared. Myasthenia gravis sera giving high titres in indirect haemagglutination test with this antigen, were selected. Rabbit antisera to CAE were prepared. The specificity of the two types of muscle antibodies was compared. Both patient sera and the rabbit antisera gave positive reactions in serological tests for muscle antibodies, but a difference in serological specificity was found.