Flow cytometric analysis of unstimulated (day 0) and PHA-stimulated PBMC (days 1 through 3). DNA staining was registered on a linear scale (FL1-A) and Ki-S2 staining on a log scale (FL2-H). Less than 1% of the unstimulated PBMC were positive. Twenty-four hours after PHA stimulation, a small population of the G₁ cells became positive. On days 2 and 3, all cells in S/G₂ and M phases bound Ki-S2.