We studied the immunohistochemical localization of Fas and nicotinic acetylcholine receptor (nAChR) in the human thymus with myasthenia gravis (MG). Thymuses were obtained from thirteen patients with MG, (thymic hyperplasia 8; thymoma 5) and six normal controls. Antibodies against Fas, nAChR protein isolated from fetal calf thymus (nAChR-FCT), CD3, CD4, and CD8 were used. In both hyperplastic thymus and controls, Fas antigen was found in medullary epithelial cells especially in those around Hassall's corpuscle, in subcapsular epithelial cells, and partially in the cortex. There were no apparent differences between myasthenic thymus and the controls. In the thymic medulla the anti-nAChR-FCT antibody showed a similar staining pattern to the antibody against Fas. The nAChR and the Fas were suggested to be present in the same cell. Then six thymomas were double immunostained with antibodies against Fas and nAChR-FCT. A part of neoplastic epithelial cells were double positive for both Fas and nAChR. The present results show the first observation on the colocalization of the Fas and nAChR in thymic epithelial cells. These antigens were supposed to work on triggering mechanism of autoimmunity in MG.