Complementary approaches with purified molecules or transfected cytolytic effector cells have suggested that both, granzyme A (gzmA) and granzyme B (gzmB), similarly contribute to CTL-mediatedand perforin (perf)-dependent apoptotic nuclear damage (DNA fragmentation) in target cells. Studies employing gzmA or gzmB single-knockout mice on the other hand indicated that gzmB is the prominent CTL effector molecule for the rapid induction of DNA fragmentation, with gzmA playing only a minor part. We have now taken ex vivo-derived virus-specific or in vitro generated alloreactive CTL from mice deficient in either gzmA or gzmB and a panel of three target cells to reinvestigate this unresolved issue. We show that rapid CTL-mediated DNA fragmentation of L1210.3 target cells is solely dependent on gzmB, whereas the DNA fragmentation of EL4.F15 target cells by the same CTL population is mainly induced by gzmA and only marginally by gzmB. Moreover, CTL-induced apoptosis of a third target cell, MC57G, was partially dependent on both gzmA and gzmB activities. The differential contribution of the two gzms to apoptosis was further verified by their distinct sensitivity tocaspase inhibitors. The data suggest that both, gzmA and gzmB, have a similar potential to induce rapid perf-mediated apoptosis but that their individual contribution to the underlying intracellular processes is dictated by the quality of the target cell.