In vitro and in vivo binding of [3H]estradiol to cytosol proteins and nuclei was studied in kidneys from normal and androgen-insensitive tfm/y mice. The use of tfm/y mice permitted study of the estrogen-receptor complex in the absence of androgen receptor. A high affinity, [3H]estradiol-labeled, 8S molecule was demonstrated in low salt sucrose gradients. This macromolecule sedimented more slowly in gradients containing KC1 (0.5 M). Binding of [3H]estradiol was inhibited by 100-fold excess estrone, estradiol, or diethylstillbestrol but was not affected by testosterone, androstenedione, or progesterone. Studies of equilibrium-binding kinetics for estradiol indicated a Kd of 1.4 X 10(-9) M and 4.4 X 10(-14) mol of binding sites/mg cytosol protein. Furthermore, the binder was an acidic, heat-labile protein (pI = 4.8) which could be precipitated from cytosol with 33% ammonium sulfate, and needed sulfhydryl groups for activity. The demonstration of an estradiol-binding protein in vitro was correlated with specific [3H]estradiol uptake by tfm/y kidney nuclei in vivo. We concluded that the mouse kidney contains an estradiol-binding protein, distinct from that for androgens, which has many of the characteristics of a steroid receptor. The presence of an estrogen receptor in both normal and tfm/y mice indicates that a genetic defect in one receptor does not influence the properties of another. We concluded that androgen and estrogen receptors are under independent genetic control.