In this report, we have studied the effect of epidermal growth factor (EGF) on the proliferation of breast cancer cell lines, as well as the modulation of estrogen -and epidermal growth factor - receptor levels by EGF treatment. We have observed that all the cell lines analysed were stimulated by EGF in low serum containing media. The MCF-7AZ cell line, its H-ras transfected MCF-7AZ TD5 variant and the MCF-7 cells, all of them containing a relatively low number of epidermal growth factor receptors, were growth stimulated in a dose-dependent manner by 10(-9) M to 10(-8) M EGF. The MDA-MB 231, A431 and BT20 cell lines that express higher receptor levels were stimulated with relatively low concentrations of epidermal growth factor (5 x 10(-13) M to 10(-11) M). However, A431 and BT20 cells were shown to be growth-inhibited in the presence of higher EGF concentrations (10(-10) M to 10(-8) M). We also observed that EGF down-regulated epidermal growth factor receptor while it up-regulated estrogen receptor. In addition, Scatchard analysis of radiolabeled EGF binding on cell surface demonstrated that the concentrations of growth factor necessary to occupy a given number of epidermal growth factor receptors are inversely correlated with the total level of these receptors. Our findings suggest that the mitogenic effect of epidermal growth factor on cell proliferation is a function of the quantity of EGF-occupied receptors.