This study describes a new method for the simultaneous assessment of the distribution of a cell population in the G0/G1, S, and G2/M cell-cycle phases by using multiparameter flow cytometry and single staining based on BrdUrd incorporation. Both the K562 cell line and PHA-stimulated peripheral blood lymphocytes (PBL) were analyzed. Cells were cultured in the presence of BrdUrd for 30 min prior to cell harvesting. Once collected, cells were exposed to ultraviolet light for 5 min and then fixed immediately in 70% ethanol (-20 degrees C) for at least 30 min. Once fixed, the cells were placed for 30 min at 37 degrees C in the presence of terminal deoxynucleotidyl transferase (TdT) and dUTP labeled with digoxigenin; they were then stained with FITC-labeled anti-digoxigenin. Our results show that G0/G1, s, and G2/M cell populations can be clearly discriminated according to FITC fluorescence and light-scatter parameters. In this way, S-phase cells can be identified by their FITC staining. From the cells which were negative for anti-digoxigenin-FITC antibody, two clear populations could be resolved in a forward scatter, side scatter, and fluorescence pulse-width three-dimensional plot; the values obtained for G0/G1 cells were lower than those obtained for G2/M cells in all three parameters. Multiparameter analysis of PBL stained for two surface antigens (CD3 and CD8) and for BrdUrd by direct or indirect TdT method permitted cell-cycle analysis of different subpopulations, including CD3+/CD8+, CD3+/CD8-, CD3-/CD8+, and CD3-/CD8-.