ZR-75-1 human mammary cancer cells can be grown in serum-free medium in an estradiol- and tamoxifen-sensitive manner. Growth occurs both in monolayer and suspension culture, although serum is necessary during the initial plating of the cells in monolayer culture. Optimal stimulation of proliferation by estradiol occurs at 10(-8) and 10(-9) M. Tamoxifen alone weakly stimulated growth in a dose-dependent manner up to 10(-7) M, but inhibition was observed at 10(-6) M. This inhibition was not seen if the cells were cultured in 10% dextran:charcoal-treated fetal calf serum. This difference is ascribed to tamoxifen binding components in the serum. In the absence of serum, tamoxifen (10(-6) M) abolished the proliferative effect of estradiol at all concentrations of estradiol below 10(-7) M. Higher concentrations of estradiol partially overcame the tamoxifen effect. Consistent antiestrogenic effects of tamoxifen required an estradiol:tamoxifen ratio of 1:1000.