To debermine the nature of the cellular immune response directed against acetylcholine receptor in myasthenia gravis, we compared lymphocyte stimulation by eel receptor with clinical factors. The mean (k SEM) stimulation index wa.s 4.5 ± 0.9 for 39 myasthenic patients and 0.97 ± 0.18 for 48 controls (p < 0.001). Positive response in patients was associated with disease onset after 50 years (10 of 13 patients) and presence of thymoma (seven of eight patients), but not with HLA type. Stimulation index correlated with disease activity (rs = 0.6 3, < 0.01). Blocking the active portion of the receptor molecule with naja toxin resulted in 68 percent diminution of the response, suggesting that this site plays a significant role in the cellular immune response in myasthenia gravis.