In vitro exposure of estrogen receptor-negative (ER-) EVSA-T human breast cancer cells to insulin and/or estradiol had no effect on cell cycle distribution, in contrast to a 3-5-fold increase in the percentages of cells in the S-phase of the cell cycle in the ER+ MCF-7 cell line. Estrogen pretreatment of MCF-7 cells followed by incubation with doxorubicin resulted in an augmented inhibition of cell growth compared to unstimulated controls. This delay in growth was accompanied by a decrease in the percentages of cells actively synthesizing DNA, and by an augmented percentage of cells exhibiting a G2M-amount of DNA at the end of a 6-9 day period of culture in complete growth medium.