Elevated proportions of monocytes have previously been found in the blood of healthy women during the ovulation period as well as in other conditions associated with increased blood estradiol (E2). This phenomenon was explained, in part, by an augmenting effect which physiological concentrations of E2 may have on the development of granulocyte-macrophage (GM) colonies derived from normal peripheral blood mononuclear cells. To analyze this effect, we tested possible alternatives for the interaction between E2, colony-stimulating factor (CSF) and GM colony progenitor cells. E2 was found not to interact synergistically with CSF, but pre-treatment of the progenitor cells with E2 resulted in higher numbers of colonies in response to CSF. Moreover, E2 did not induce higher secretion of CSF but treatment with anti-CSF antibodies abolished the enhancing effect of E2. Based on these results, we suggest that the augmenting effect of E2 on GM colony formation is mediated by inducing the colony precursor cells to be more responsive to CSF. These findings may help to elucidate some of the complex relationships between estrogens, immune responses and hemopoiesis.