A purified citric acid soluble extract from human skeletal muscle (AEMA) and a phosphate-buffered saline extract from rabbit muscle acetone powder (EMA) were used to coat polystyrene beads in an enzyme-linked immunosorbent assay (ELISA). From 54 patients with myasthenia gravis, positive results were observed in 14. Five of 6 patients with thymoma had high levels of antibodies. With the diagnostic difficulties in detecting small and medium-sized thymoma, a sensitive assay for detection of antibodies to muscle antigen may be an important supplementary tool to detect tumors at early stages.