Tamoxifen is known to inhibit the growth of some human mammary carcinoma cells; this effect is accompanied by a decrease in the proportion of cells synthesizing DNA. In this work, flow cytometry of DNA and of bromodeoxyuridine labeling and the evaluation of the cell cycle-related antigens Ki-67, PCNA, and statin were used to investigate the changes in the proliferation kinetics of MCF-7 cells before and after treatment with 10(-7) M TAM. The treatment with TAM induced a significant decrease in the fraction of S-phase cells and an increase in those with a DNA content typical of G0/1 phase. The TAM-induced block in G0/1 is paralleled by a decrease in the frequency of cells expressing Ki-67 and PCNA, and by an increase in statin-positive (G0) cells. These results confirmed that the TAM-induced inhibition of cell growth is associated with major changes in the cell cycle parameters of MCF-7 cells, and provide the first experimental evidence that two main mechanisms are operating: the accumulation of cells in G1, before the onset of S-phase, and the exit of some cells from the cycling compartment.