A study was conducted to compare the biological activity of the estrogenic component of the endogenous steroids in breast milk samples collected during various phases of lactation with those milk samples collected from women who were on estrogen therapy. The estrogenic biological activity in the milk sample was assessed by the immature mouse uterine weight gain assay. Milk samples collected during postpartum period from six different study groups, viz., control colostrum of 1-3 days and 4-6 days, transitional milk (10-20 days) and mature milk (1-3 months) were compared with colostrum and mature milk of women treated with Lynoral (ethinyl estradiol 0.1 mg) three times a day for three days. Estrogenic activity was observed only in animals injected with milk extracts of colostrum samples from both control and Lynoral-treated women; however, they were not significantly different from each other. Therefore it is not the exogenous estrogens, but the endogenous estrogen present in large quantities in the colostrum that is responsible for the biological activity.