Anti-oestrogens exert a tumoristatic effect on estrogen-receptor-positive breast carcinomas in vivo. At a cellular level this may reflect inhibition of cell proliferation and/or cell death counterbalanced by continued proliferation of a cell subpopulation. We evaluated the MCF-7 human mammary adenocarcinoma cell line as an in vitro model to study the effects of the novel oestrogen antagonist ICI 182,720 on cell population dynamics (cell gain vs. cell loss). After oestrogen-withdrawal monolayer cell number declined over 10 days, accompanied by cell detachment. This decrease in viable cell number was elevated 2-fold by ICI 182,780. Detached cells exhibited DNA fragments of 50 and 300 kbp, typical of apoptotic cells. However, internucleosomal cleavage to 180 bp integer fragments was not seen, and these detached cells exhibited a morphology which was not consistent with apoptosis. The remaining attached monolayer cells were morphologically viable (> 99%) with regard to both nuclear morphology and plasma membrane integrity. There was no difference in cell cycle phase distribution between oestrogen-withdrawn and ICI 182,780-treated cells; both induced accumulation in G1 phase. MCF-7 cells were also exposed to a variety of DNA damaging agents known to induce apoptosis in other cell types. We could demonstrate only limited induction of morphologically recognisable apoptosis in MCF-7 cells treated with methyl methanesulphonate. Our results add to the controversy surrounding the ability of the MCF-7 cell line to undergo apoptosis in vitro in response to anti-oestrogen therapies.