The antiacetylcholine receptor antibody was titered in the serum of 63 patients with myasthenia gravis (MG) and 20 control healthy subjects. The titer was significantly high in 92% of MG patients in contrast with none of the controls and no correlation was found with the thymus pathology and the severity of the disease. The titer decreased after thymectomy almost steadily with the improvement of the myasthenic signs. The role of the antibody in the pathogenesis of the disease is discussed.