Influence of estradiol and tamoxifen on the growth of N-nitrosomethylurea-induced rat mammary tumor cells in soft agar.

N-Nitrosomethylurea-induced rat mammary tumors were grown in vitro using the clonogenic soft agar technique. Cells from all tumors (n = 46) formed colonies in vitro. Tamoxifen (10(-7), 10(-6) M) inhibited colony formation to 72 and 53% of control values, respectively. The inhibitory effect of tamoxifen could be reversed with the addition of estradiol (10(-10) to 10(-8) M) to the medium. Estradiol (10(-10) to 10(-8) M), on the other hand, added alone to serum-containing medium did not influence the number of colonies formed in vitro. We conclude that the soft agar clonogenic assay is a feasible technique to study the influence of hormones and antihormones in vitro. The effects of tamoxifen and estradiol noted in vitro were similar to the known in vivo effects of these agents.