Long-term treatment of the estrogen sensitive human breast cancer cell line EFM-19 with the antiestrogenic compound Tamoxifen resulted in a variant line EFM-19 T, which was stimulated by Tamoxifen. Estrogen receptor analysis by radioligand assay (charcoal method), revealed a 2.5 fold higher receptor concentration in EF-19 T cells than in the parental EFM-19 cell-line. As demonstrated with the immunocytochemical assay (ER-ICA) only 60% of the parental EFM-19 cells were estrogen receptor positive, whereas 98% of the EFM-19 T cells expressed estrogen receptor protein. In addition, receptor content per cell was higher in the Tamoxifen treated subline than in the parental cell line. Analogous with the growth promoting effect of Tamoxifen on EFM-19 T cells, Tamoxifen acted like estrogen leading to a down regulation of cellular estrogen receptor concentration. The partial growth dependency of the EFM-19 T cells on the presence of Tamoxifen demonstrates estrogenic effects of Tamoxifen and explains the withdrawal response obtained in the treatment of breast cancer patients when remission occurs after termination of ineffective treatment with Tamoxifen.