Current studies have been primarily concerned with demonstrating differences among mammary tumor estrogen receptors which differ in their response to ovariectomy. It has been demonstrated that the normally non-responsive tumor (MtT-supported) contains an inhibitory substance in its cytosol preparations. One of the hormonally responsive tumors (spiramide supported) also contains an inhibitory substance in its cytosol preparations. The third tumor line (domperidone-supported) does not contain inhibitory substances in cytosol preparations. The amount of estradiol receptor binding to DNA-cellulose decreases with increasing concentration of estradiol receptor from both the MtT and spiramide supported tumors. The domperidone supported tumor, however, does not show decreased binding at high receptor concentrations. Inhibitor substance prepared from MtT and spiramide tumor will inhibit DNA-cellulose binding reciprocally. Domperidone estradiol receptor is not affected by any of the "inhibitor" preparations. The presence of an inhibitory substance has also been demonstrated in human mammary tumor specimens. 125I labeled estradiol has been used in human studies enabling sufficient radioactivity level to be obtained in small samples due to its very high specific activity compared to tritiated estradiol.