Bio-assay by the Hooker-Forbes method for progestin activity of mixtures of progesterone with estrone and with 17[beta]-estradiol revealed complete inhibition, partial inhibition, no interference, or synergism, depending on the proportions of the mixtures. Bio-assay of progesterone:estriol mixtures indicated partial or complete antagonism. These mechanisms may exercise significant control of the action of progesterone.