To study the activation of T lymphocytes in hyperthyroid Graves' disease, the serum concentrations of soluble interleukin 2 receptors (sIL2R) were determined during active thyrotoxicosis and following the return to a euthyroid state with carbimazole therapy. Serum sIL2R was measured by an enzyme linked immunoassay. The mean +/- SD serum sIL2R concentration during untreated hyperthyroidism was elevated as compared with controls (919.1 +/- 523.4 vs 374.2 +/- 189.4 U/ml, P less than 0.005). However, after carbimazole therapy the serum sIL2R in euthyroid patients fell to 377.9 +/- 90.3 U/ml, which did not differ from healthy controls. Serum sIL2R correlated significantly with the serum free T3 only during hyperthyroidism (r = 0.678, P less than 0.01). Our study suggests that in vivo measurement of serum sIL2R released from activated T lymphocytes is a useful immunological indicator of disease activity.