Three radiohalogenated derivatives and two tritium labeled estrogens with a range of affinity constants were evaluated as radio indictors for estrogen receptors. These receptor binding radiotracers may be useful in the detection of estrogen dependent malignancy. In vitro receptor assays using immature rat uterus showed that the radiolabeled estrogens had receptor affinities ranging from 10 to the 7th power/M to 2 times 10 to the 9th power/M. All concentrated in the uterus of the immature rat; however, based on in vivo displacement studies using excess estradiol, only H-3 estradiol (E), Br-77 bromoethynylestradiol (BEE), and Br-77 bromoethynyl, 11-methoxy estradiol (BEME) showed estrogen receptor binding. I-125 iodohexestrol showed a reported high affinity constant in vitro but nonspecific binding competed effectively with receptor binding in vitro. Monoiodohexestrol exhibits 10 to 15% specific binding to the 8S estrogen receptor while the remainder binds to nonreceptor 4S protein. Reduction of nonreceptor binding with either thyroxine or 8-anilino-1-naphthalene sulfonic acid was not quantitative. Thus no accurate determination of the concentration of receptor sites in the radioreceptor assay was possible by graphical analysis. When the displacement study was carried out in vivo with thyroxine, receptor binding was detected. No receptor binding was detected with the antiestrogen CI 625. These results agree qualitatively with a bimolecular model which predicted uterus to blood (U/B) ratios of greater than 1 for all compounds except the antiestrogen. As the model predicts, specific activity plays an important role in the U/B ratio. BEE which had a specific activity of less than 10 C/mmol had a lower U/B ratio than E which had a specific activity of 100 C/mmol. Further experiments using E at various specific activities confirmed these results. The BEE and BEME are both stable in vitro and in vivo to debromination. Two of the gamma emitting estrogen derivatives BEE and BEME should be useful as probes for estrogen dependent malignancy.