Clinical and experimental observations suggest that oestrogens may influence immune reactivity. The oestrogen-binding properties of the human thymus were studied by the use of an aqueous two-phase separation technique. A specific, high-affinity and low-capacity binding was found in 9 out of 10 thymic tissue samples obtained from children and adults of both sexes, their ages ranging from 1 to 42 years. The association constant ( ± SD) for the oestradiol binding in human thymus was 3.7 ± 1.4 × 109M-1 and was in the same range as the receptor binding in the human uterus. The steroid specificity was found to differ from the classical oestrogen receptor. The concentration of oestrogen-binding sites ( ± SD) was 182 ± 181 fmol/mg protein and was higher than the receptor concentration of the uterus. According to histochemical analyses the binding is located in the reticulo-epithelial stroma rather than the thymocytes. Data suggest that the human thymus is a target organ for oestrogens which may influence lymphocyte function during therapy.