Circulating immune complexes were assayed employing the method recently described by Barnett and Chia in a group of patients with myasthenia gravis. The subjects were classified according to clinical severity and immune complexes were sought before and after thymectomy. The operated subjects were further divided into those with thymoma or thymic hyperplasia. Antigen-antibody complexes were higher before thymectomy than after, in hyperplasias than in thymomas, and in severe myasthenia gravis than in mild disease. Circulating immune complexes and anti-acetylcholine receptor antibodies did not correlate.