The effect of 10(-6) M tamoxifen on MCF-7 cells adapted to growth in 0.5% foetal calf serum has been studied. The growth inhibitory effect of this tamoxifen concentration was abolished by simultaneous addition of 10(-8) M oestradiol, indicating that tamoxifen may exert its effect via binding to the oestrogen receptor. Flow cytometric cell cycle analysis of tamoxifen-treated cultures showed an increase in the proportion of cells in the G1 phase of the cell cycle. By exposing the cells to BUdR before flow cytometry the growth fraction was determined and found to be dramatically decreased in tamoxifen-treated cultures. Cells were not only arrested in the G1 phase but also in the G2 phase of the cell cycle. A few colonies of MCF-7 cells were resistant to 10 days treatment with 10(-6) M tamoxifen.