We studied 11 patients with myasthenia gravis who demonstrated a cellular immune response to acetylcholine receptor (AChR) of the electric organ of Torpedo marmorata. After thymectomy, there was a marked decrease in the patients' lymphocyte reactivity to AChR. The mean reduction of the stimulation index (SI) was 50 percent, but the response to the nonspecific mitogen phytohemagglutinin (PHA) was not affected. In six cases, the lymphocyte response was measured at intervals up to 22 months after thymectomy; in all six, the immune response to AChR remained decreased. In some cases, the response continued to decrease, even to normal values. The effect of corticosteroid treatment was tested in other patients. The cellular immune response to AChR was significantly lower in treated patients (mean SI, 1.64 ± 0.25) than in untreated controls (mean SI, 2.41 ± 0.38), with no significant difference in the response to PHA. These data suggest that a decrease in the cellular immune response to AChR may be one mechanism by which thymectomy and corticosteroids are therapeutic in myasthenia.