We have utilized the estrogen receptor (ER)-positive human breast carcinoma cell line, T47D, to determine the role of ER in regulating cell proliferation, the level of expression of p53 and the state of phosphorylation of retinoblastoma protein (pRB) by 17 beta-estradiol (E2) and antiestrogens. T47D cells cultured for 7 days proliferated rapidly expressing maximal levels of p53 in medium containing 5% fetal bovine (whole) serum. Exogenously added E2 had no effect on either of the above parameters. The antiestrogen, ICI 164,384 (ICI, 1 microM), decreased cell number and p53 level to nearly 20% of the control. Comparatively, a treatment of the cells with 100 nM 4OH-tamoxifen (OHT) decreased cell number to 40% of the control without a concomitant decrease in the p53 levels suggesting a differential ability of these antiestrogens to regulate p53 levels in cells cultured in whole serum. When cells were cultured in medium containing serum depleted of endogenous steroids (charcoal stripped serum), cell number and p53 levels declined. Treatment with exogenous E2 (1 nM) increased cell proliferation, p53 expression and phosphorylation of pRB. The antiestrogens ICI and OHT blocked these E2 effects, demonstrating a direct antagonism of ER by ICI and OHT. These results indicate an ER-mediated mechanism for coordinate expression of p53 and hyperphosphorylation of pRB during E2-induced proliferation of T47D cells.