We tested thymus cells from 119 patients with acquired myasthenia gravis (MG) for in vitro production of acetylcholine receptor (AChR) binding antibodies (Ab); 109 were seropositive, of which 82 (75%) secreted AChRAb in vitro. As noted in earlier studies, thymus cell secretion of AChRAb paralleled serum Ab levels (rho s = 0.503; p < 0.0001; n = 119). Striational Ab secretion also correlated with the patients' serologic status. AChRAb secretion in vitro predicted the secretory activity of thymus cells implanted in severe combined immune deficiency (SCID) mice. Thymocytes from patients treated with corticosteroids made significantly less AChRAb than thymocytes from untreated patients (p < 0.005). Of particular note: (1) AChRAb was secreted by thymocytes from two of five nonimmunosuppressed patients who had generalized MG but who were seronegative for AChR binding autoantibodies; and (2) AChRAb secretion from both thymic and nonthymic sources was documented in a patient with noninvasive thymoma whose first signs of MG appeared 7 months after thymectomy. We conclude that the thymus in a majority of patients with acquired MG is the principal but not the sole reservoir of immunocytes that specifically react with muscle antigens.