The lymphocyte subpopulations were detected in the target organs obtained from various autoimmune disorders on the tissue cryosections. Thymus-derived lymphocytes (T cells) and bone marrow-derived lymphocytes (B cells) were detected by rosette formation with sheep erythrocytes (E) or with sheep erythrocytes coated with antibody and complement (EAC) respectively. Almost all infiltrating lymphocytes in the salivery glands in patients with Sjogren's syndrome were identified as B cells and T cells were detected only along the ducts. In Hashimoto's thyroiditis, infiltrating lymphocytes with the lymph follicles in the thyroid gland were mostly B cells whereas T cells localized as surrounded the thyroid follicles. T cells would not be detected in the spleen sections obtained from the patients with hypoplastic anemia at all. The white pulp was constituted by B cells, and the mononuclear cells in the red pulp might be macrophages of which these cell surfaces had IgG-Fc receptors. The lymph follicles of the thymus in a patient with myasthenia gravis complicated with thymoma was demonstrated to be formed with B cells. Therefore, it could be concluded that B cells were predominant lymphocytes in the target organs, while T cells localized in situ developing the autoimmune reactions in various autoimmune diseases.