The estrogen receptor positive human breast cancer cell line, MCF-7, can be growth inhibited by high concentrations of newborn calf serum (NCS). MCF-7 cells grown with high concentration of NCS can be growth stimulated by 10(-8) M estradiol, and the growth stimulation seems to involve the abolishment of the effect of inhibitory activity in serum. Flow cytometry has been used to determine cell cycle parameters such as distribution of cells in the different phases of the cell cycle and growth fraction. The cell cycle analysis revealed that addition of 10% NCS to cultures grown with 0.5% fetal calf serum (FCS) increased the doubling time by elongating the G1 transit time. Estradiol stimulation occurred through a shortening of the G1 transit time. An effect on growth fraction was observed neither during growth inhibition with high NCS concentration nor during growth stimulation with estradiol. Cells which are growth stimulated by estradiol have an activated estrogen receptor mechanism as indicated by the presence of filled nuclear estrogen receptors and high level of progesterone receptors. We suppose that a possible mechanism for this estrogen stimulation could be induction of synthesis of growth factors which annual the effect of the inhibitory activity present in NCS.