The effects of tamoxifen on cell cycle progression and clonogenic survival were examined using synchronized cultures of MCF-7 human mammary carcinoma cells. Cell synchrony was induced by mitotic selection. Subsequent cell cycle analyses, using DNA flow cytometry, showed that 85% of synchronized cells had a mean cell cycle time of 21.3 h with mean phase durations of 9 h for G0-G1, 9.3 h for S, and 3 h for G2 + M. A slowly cycling or noncycling subpopulation comprising 15% of the total population was also observed. Exposure to tamoxifen (5-12.5 .mu.M) resulted in a dose-dependent reduction in the number of cells progressing through G0-G1 and entering S phase. Those cells which were not retained in G0-G1, however, appeared to traverse G0-G1 and the remainder of the cell cycle at a rate only slightly less than that of untreated controls. Further experiments demonstrated that the major sensitivity to tamoxifen in terms of both inhibition of cell cycle progression and drug cytotoxicity was restricted to a short interval in the middle of G0-G1. This 2-4 h period of maximum drug sensitivity began .apprx. 4 h after mitotic selection, with drug exposures outside this time frame having markedly fewer effects.