Biological assay methods for estrogens and androgens are in great demand and new improved methods are urgently needed in the development, screening, testing, and control of estrogenic and androgenic materials. More rapid, sensitive and specific bioassay methods are of interest if they reduce the time and cost of the bioassays and extend the sensitivity of detec¬tion.nExperiments reported herein were carried out to test the usefulness of selected isotopes as indicators of estrogenic or androgenic activity. The nuclides Na22 , Zn65 , I131, and p32 were employed in tissue uptake studies in response to graded estrogenic stimulation in both adult-ovariectomized and immature rats. Of the four isotopes tested, Na22 demonstrated the most useful log-dose response relationship when uterine Na22 of immature rats was measured. The assay can be performed in six hours and its ability to detect an unknown amount of estrogenic material was demonstrated. The method was found to be specific in the presence of progesterone, but cortisone altered the Na22 uptake of the uterus. The range of estradiol detection was between 0.025 - 0.2 micrograms.
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