Two types of endocrine therapy that have been successfully applied to patients with hormone-dependent breast cancer are the non-steroidal antiestrogen tamoxifen, and inhibitors of aromatase, the enzyme that synthesizes estrogens. The major drawback with tamoxifen is that it acts as a partial estrogen-agonist and this is believed to mediate, at least in part, acquired tumor resistance to the drug as well as endometrial hyperplasia and carcinoma in some patients. The newer and more potent antiestrogen ICI 182,780 is a steroidal molecule that is devoid of estrogenic activity. We now report that ICI 182,780 is also an inhibitor of aromatase activity in fibroblasts isolated from the normal human breast as well as other carcinoma cell lines that express aromatase (MCF-7Ca breast cancer and JEG-3 choriocarcinoma). ICI 182,780 (1 microM) did not affect aromatase activity levels in human placental microsomes and only inhibited aromatase activity in each of the cell lines following a prolonged incubation period. In the fibroblasts, inhibition of aromatase activity by ICI 182,780 was shown to be time and dose-dependent. In contrast, tamoxifen and 17beta-estradiol were shown to have no effect on aromatase activity levels. ICI 182,780 inhibited aromatase activity levels with IC50 values of 16.80 nM in MCF-7Ca cells, 125.50 nM in JEG-3 cells and 386.1 nM in breast fibroblasts. These values were compared to those for known aromatase inhibitors, and in each of the cell lines the order of potency was letrozole>4-OHA>anastrozole>ICI 182,780. The inhibition of aromatase activity by ICI 182,780 was sustained even after the antiestrogen was removed from the cells indicating that ICI 182,780 may be remaining bound to the enzyme. Although ICI 182,780 had no effect on the proliferation of the fibroblasts, or JEG-3 cells, it significantly inhibited the growth of MCF-7Ca cells. This growth inhibition appeared to be due to the antiestrogenic activity of ICI 182,780 and not to its aromatase inhibiting effects. ICI 182,780 did not inhibit aromatase activity by down-regulating levels of the aromatase transcript. These results show that in addition to being a potent antiestrogen, ICI 182,780 is also an inhibitor of cellular aromatase activity, and suggest that by interfering with the actions of estrogen by two distinct mechanisms, ICI 182,780 may be a suitable drug for treating patients with hormone-dependent breast cancer.