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Twelve biopsied thymuses taken from 4 cases with myasthenia gravis (MG group) and 8 cases without myasthenia gravis (control group) including 2 thymoma cases in each group were immunopathologically investigated in relation to myoglobin (Mb). Mb positive cells of various degrees were detected in all thymuses of both groups, and immunoelectron microscopical examination disclosed that Mb positive cells corresponded to interdigitating reticulum cells and myoid cells in non-neoplastic thymuses, and neoplastic epithelial reticular cells in thymomas. Anti-Mb antibody staining by direct immunoperoxidase technique revealed positive localization to the lymphoid cells in the thymuses of 2 cases of MG group with thymoma. In addition, indirect immunofluorescent study with the serum of each case which was applied to the normal human skeletal muscle, showed positive staining of the sarcoplasm in 3 cases of MG group, including 2 thymoma cases, and using peroxidase labeled serum IgG F (ab')2 of the same patients this anti-muscle antibodies were proved to be against both postsynaptic cytosol and sarcoplasm of the extraocular muscle of the guinea pig. From these results, it was suggested that Mb may conduct itself as a homologous antigen between the thymus and the skeletal muscle in the myasthenic patient with thymoma, and in the thymus the interdigitating reticulum cell, the myoid cell, or the neoplastic epithelial reticular cell may retain or produce Mb as an antigen-presenting cell.