We reported recently that roscovitine arrested human MCF-7 cancer cells at G2-M phase of the cell cycle and concomitantly induced apoptosis. After roscovitine treatment, the level of wild-type p53 protein strongly increased and p53 was accumulated in the nucleus. Here, we raised the question of which pathway would be involved in roscovitine-induced apoptosis in MCF-7 cells, which are known to be caspase-3-deficient, and whether roscovitine-mediated activation of p53 protein might positively affect the execution of cell death. Roscovitine induced a depolarization of mitochondrial potential beginning at 6 hours posttreatment as evidenced by changes in J-aggregate formation and release of the mitochondrial proteins cytochrome c and apoptosis-inducing factor. Interestingly, roscovitine stimulated a site-specific phosphorylation of wild-type p53 protein in a time-dependent manner. p53 protein was specifically phosphorylated at Ser46. P-Ser46-activated wild-type p53 tumor suppressor up-regulated p53AIP1 protein, its downstream target known to mediate the depolarization of mitochondria. The onset of phosphorylation of p53 at Ser46 preceded the up-regulation of p53AIP1 protein and the depolarization of mitochondrial potential. We compared the kinetics of roscovitine-mediated p53 activation between caspase-3-deficient parental MCF-7 cells and cells reconstituted with caspase-3. The kinetics and the extent of p53 protein activation in caspase-3-proficient cells differed from those observed in caspase-3-deficient parental cells. Remarkably, roscovitine failed to induce phosphorylation at Ser46 in caspase-3-reconstituted MCF-7 cells. Our results indicate that, depending on the status of caspase-3 in MCF-7 cells, different apoptotic pathways were initialized.