: We measured soluble interleukin 2 receptor, a part of the Tac protein (p55), in peripheral blood to study the immunological condition of the T cell in autoimmune thyroid disease. In 26 patients with untreated Graves' disease and 7 hyperthyroid patients with Hashimoto's thyroiditis, the mean levels of soluble IL-2 receptor were both significantly higher than in normal controls (1497±649 (mean ± sd), 641±137 vs 221±63 103 U/l, p3 was administered to subjects in remission from Graves' disease and in normal controls, the soluble IL-2 receptor levels significantly increased. Moreover, the mean level of soluble IL-2 receptor in patients with toxic multinodular goitre was also significantly higher than in normal controls (411±148 vs 221±63 103U/l, p<0.05). We conclude that the soluble IL-2 receptor levels are higher in sera of subjects with elevated levels of thyroid hormone.