A specific receptor for progesterone has been found in a cystosarcoma phyllodes, as determined by charcoal adsorption and sucrose gradient analysis. Similar assays for estrogen receptors were negative. The tumor consisted almost entirely of stroma that contained the progesterone receptors. The epidemiology and natural history of cystosarcoma do not strongly support the hypothesis that it is controlled by female sex hormones, but the presence of the progesterone receptors suggests that some cystosarcomas are hormonally regulated, and thus may be responsive to therapeutic hormonal manipulation.