Six laryngeal squamous cell carcinoma (SCC) cell lines and the MCF-7 breast cancer line, all of which contain estrogen and progesterone receptors, were tested for in vitro growth inhibition by the antiestrogen tamoxifen citrate. Cell line MCF-7 was more sensitive to growth retardation at 1 mumol/L of tamoxifen citrate, but SCC lines showed higher sensitivity than MCF-7 to cytotoxic effects at higher drug concentrations (7 to 10 mumol/L). A cytostatic level of growth inhibition was produced by 5 mumol/L tamoxifen citrate for both types of cell lines. When tamoxifen and estradiol were added to cultures simultaneously, partial reversal of growth inhibition was observed with MCF-7 but not with UM-SCC-5. All of the cell lines recovered from inhibition when tamoxifen was replaced with 0.1 mumol/L estradiol, but laryngeal SCC lines recovered equally well in estradiol-free medium, whereas MCF-7 recovered only partially in the absence of estradiol. These findings indicate that there are at least two tamoxifen receptor sites--one for which estradiol is a competitor and one for which it is not--and that tamoxifen reversibly blocks growth and may synchronize SCC cells.