In previous publications (4, 5) we have demonstrated that ACTH and cortisone will enhance the inhibitory effect of radiation on tumors in mice. Various authors have suggested that androgens and estrogens may have similar effects. Nathanson (3) observed that with administration of estrogens more intense radiation reactions were obtained in human breast tumor, as well as in the surrounding skin, than with a comparable radiation dose without the hormone. Graham and Graham (2) reported that both testosterone and stilbestrol increased the sensitivity of carcinoma of the cervix to radiation as determined by the response of cells in vaginal smears. Both androgens and estrogens affect the resistance to total-body irradiation. Testosterone increased the mortality in the female, stilbestrol in the male, while estradiol decreased the mortality of male mice (1). Rugh and Wolff (6) observed an increase in resistance to a lethal dose of radiation in castrate male mice given estradiol benzoate. Spellman, Carlson, and Lillehei (7) found that testosterone propionate enhanced the mortality, while long-acting testosterone cyclopentylpropionate decreased the mortality of male mice following total-body exposure. Advanced salivary-gland tumors in man were favorably affected by estrogen therapy alone or in combination with x-ray therapy in small amounts (8). Material and Methods The following tumors were implanted in the anterior chamber of the mouse: S37 sarcoma in ABC strain C1300 neuroblastoma in ABC strain 15091 breast adenocarcinoma in A strain E0771 breast adenocarcinoma in C57B1 strain On the fourth day after transplantation 3,000 r∕air was administered through a field of 0.8 cm., surrounding the tumor-bearing eye. Survival at seventy days of animals receiving hormone and x-ray irradiation was compared with the survival of animals treated by x-rays alone. Estrogen in the form of estradiol 17-beta cyclopen tylpropionate (Depo-Estradiol) was administered, starting two weeks prior to or on the day of tumor transplant and continued for seventy days in the following doses: 50 micrograms per week, 25 micrograms per week, 25 micrograms every two weeks. The solvent, cottonseed oil, did not produce any noteworthy effect on tumor growth and survival of the animals. Since no significant difference was observed between various dose levels of estradiol, the groups receiving various amounts were pooled. Testosterone was administered in similar manner, as the long lasting cyclopentylpropionate (Depo-Testosterone) in doses of 10 mg. per week, 5 mg. per week, or 5 mg. every two weeks. Results Administration of estrogens produced a significant decrease in survivals (Chart I), apparently reducing the inhibiting effect of x-ray on tumor growth in male and female ABC mice with 15091 breast tumor as well as in male ABC mice with S37 sarcoma.