In Hashimoto's thyroiditis, not only a profuse lymphocytic infiltration but also macrophages infiltration are seen in the thyroid gland. In this paper, the immunopathological roles of the monocyte-macrophage cell line in Hashimoto's disease was reported. By the histological examination of the thyroid glands from 39 patients with Hashimoto's thyroiditis, infiltrations of macrophages were present in the thyroid follicles of 12 out of 23 patients with diffuse thyroiditis and of 13 out of 16 patients with focal thyroiditis. In these follicles with macrophage infiltrations, acinar cells showed eosinophilic changes characteristic of Hashimoto's thyroiditis and disappearance of follicular colloid. It was not known whether such macrophages were nonspecific scavenger cells or specifically activated cells against thyroid antigens and involved in the immunologically specific tissue destruction in Hashimoto's disease. Whether peripheral monocytes, precursors of tissue macrophages, in patients with Hashimoto's disease have increased ability to bind thyroid specific antigen (thyroglobulin) or not was studied by rosette formation with thyroglobulin coated RBC (E-Tg) in vitro. Monocytes binding E-Tg were increased significantly in 7 patients out of 14. Percentage of monocytes binding E-Tg were from 1.2 to 4.6% in Hashimoto's patients and 0-0.4% in 7 normal controls. Such specific antigen binding capacity of monocytes in Hashimoto's disease was thought to be due to the presence of cytophilic antibodies on the surface of monocytes. Monocyte cytophilic anti-thyroglobulin antibodies were detected in 24 sera out of 30 patients with Hashimoto's disease. Monocytes from normal healthy donors obtained the ability to form rosette with E-Tg on incubating them with heat-inactivated Hashimoto's sera for 60 min at 4℃. After thorough washing, such monocytes formed rosette with E-Tg in vitro. The rosette formation was immunologically specific and did not form rosette with erythrocytes coated with unrelated antigens such as human serum albumin. The rosette formation was inhibited by the addition of small amount of free thyroglobulin in the medium. Cytophilic activities of sera (% rosette) were correlated with anti-thyroglobulin antibody titers by passive hemagglutination. It was suggested that the monocytes arming with such cytophilic antibodies in vivo might be responsible for specific antigen binding capacity of monocytes in patients with Hashimoto's disease and such armed monocytes may infiltrate in the thyroid gland showing specific affinity to colloidal antigens. Penetration of such cells into the colloid follicles through acinar cells might disrupt the follicular architecture and might play a part of thyroid tissue damage.