An experimental mouse model of autoimmune thyroiditis induced by neonatal thymectomy (between 2 and 4 days of age) is described. Thymectomy within 24 hours after birth or thymectomy after 5 days of age failed to induce the disease. This type of thyroiditis occurred spontaneously a few months after the operation, lasted until approximately 12 months of age, and appeared to subside with aging. Hybrid mice of (C3H/He X 129)F1 are so far the most susceptible to neonatal thymectomy and the incidence in females (25 per cent) is significantly higher than that in males (6 per cent). Mice with thyroid lesions usually had circulating antibodies to mouse thyroid extract. Histologically, lymphocytes and plasma cells were the main cellular types of infiltrates and lymphoid follicles were seen in some cases. Interestingly, adenoma-like nodular hyperplasia of thyroid epithelium was observed in several thymectomized mice, with or without lymphoid infiltration. Besides those observed in the thyroid gland, lymphoid infiltrations were often observed in such organs as the ovary, the stomach, and the coagulating gland of thymectomized mice. These characteristics are compared with those of Hashimoto's thyroiditis and with those of its other animal models, and possible mechanisms which may be involved in the present disease are discussed.