The cellular immune response to acetylcholine receptor from Torpedo electric organ was studied in 100 myasthenic patients and 41 healthy subjects. The mean stimulation index (SI) was 2 ± 0.15 for the patients, and 1.06 ± 0.08 for the controls. Stimulation was significantly greater when the test medium contained autologous serum rather than a standard universal serum (AB serum). Young patients were generally good responders (SI, 2.39 ± 0.26), but older patients usually did not respond (mean SI, 1.18 ± 0.13). Among the younger patients, men had higher responses than women (mean SI, 3.13 ± 0.63 and 2.05 ± 0.23, respectively). There was no correlation between degree of lymphocytic reactivity and duration or severity of symptoms.