The MCF-7 human breast cancer cell line responds to 5-androstene-3 beta,17 beta-diol, to dehydroepiandrosterone and to its 3 beta-sulfate stimulation in vitro by increased proliferation. 5-Androstene-3 beta,17 beta-diol has the strongest mitogenic effect. Dehydroepiandrosterone has a more potent action on cell proliferation than its 3 beta-sulfate. These adrenal androgens do not stimulate proliferation of the estrogen receptors negative for the BT-20 cell line. In our conditions, we have failed to demonstrate MCF-7 estradiol formation when incubations were carried out in the presence of labeled adrenal androgens. These results suggest that adrenal androgens stimulate proliferation of MCF-7 cells directly by acting on estrogen receptors.