Flow microfluorometric analysis of human lymphoid cells exposed in vitro to cytostatic concentrations of podophyllotoxin (0.01-5 mug/ml for 24 h) shows that a major part of this population (40-60%) has the DNA content of cells in the G2-M part of the cell cycle, and that approximately 60% of these cells are arrested in mitosis. Although a similar pattern of DNA distribution is seen in cultures exposed to cytostatic concentrations of VM-26(0.01 mug/ml) and VP--16-213(0.1 mug/ml), no mitotic cells are seen in these cultures. Exposure to higher concentrations: of VM-26 (0.1 mug/ml) and VP-16-213 (1.0 mug/ml) inhibits cell cycle traverse, and after 24 hr of exposure a major part of the population is arrested with the DNA content of cell in the S part of the cell cycle. Exposure to higher drug concentrations leads to a reduction in the number of cells with the late S-G2DNA content. Whereas the cell cycle block induced by cytostatic concentrations of podophyllotoxin (0.01 mug/ml) is readily reversible by reincubation of cells in drug-free medium, cells blocked by VM-26 and VP-16-213 are unable to resume cell-cycle traverse under similar conditions.