MCF-7 human breast cancer cells, grown in long-term tissue culture, were found to be highly responsive to prolactin in terms of growth even in the presence of serum. Human prolactin, placental lactogen, and growth hormone (50-250 ng/ml) stimulated MCF-7 cells to grow when added to culture medium of cells in the presence of charcoal-stripped serum. Within 3 days of the hormone addition, a 4.4-fold increase in cell number was achieved with human prolactin at 100 ng/ml in the presence of 10% charcoal-stripped serum. Under these same conditions, estradiol-17 beta at 10(-8) M achieved only a 2-fold increase. After 6 days of culture, both estradiol-17 beta and prolactin gave a total 5-fold increase in cell number. No prolactin effect was achieved in the presence of 10% fetal bovine serum. Stripping fetal bovine serum with dextran-coated charcoal removes as much as 85% of the endogenous lactogens. Removal of these hormones is essential for demonstration of subsequent prolactin-induced growth response in MCF-7 cells, since bovine prolactin binds effectively to lactogen receptors on the surface of the cells but does not transmit a growth signal. When added simultaneously with human prolactin, bovine prolactin blocks the growth response to the former hormone. These results clearly demonstrate that, under the proper conditions of culture, the human breast cancer cell line MCF-7 is highly responsive to growth stimulation by homologous lactogenic hormones. This then affords us an excellent model for further studies on the possible role of prolactin in growth and maintenance of human breast cancer.