Caspase-3 activity has been described to be essential for drug-induced apoptosis. Recent results suggest that in addition to its downstream executor function, caspase-3 is also involved in the processing of upstream caspase-8 and -9. To test the absolute requirement for caspase-3, we examined mitomycin C (MMC)-induced apoptosis in the caspase-3 deficient human breast cancer cell line MCF-7. MMC was used as anticancer drug since this agent was preferentially active compared to chemotherapeutic compounds with differing mechanisms of action such as cisplatin, docetaxel, or lovastatin. MMC treatment led to pronounced caspase-8, -9, and -7 processing and early morphological features of apoptosis within 48 h. This could be inhibited by the broad-spectrum caspase inhibitor z-VAD.fmk and to a lesser extent by z-IETD.fmk and z-LEHD.fmk, which have a certain preference for inhibiting caspase-8 and -9, respectively. MMC induced apoptosis in MCF-7 cells was not mediated by the death receptor pathway as demonstrated by experiments using the inhibiting anti-Fas antibody ZB4 and transfections with CrmA, a viral serpin inhibitor of caspase-8, and the dominant negative Fas-associated death domain (FADD-DN). Stable expression with Bcl-2 significantly prevented the processing of caspase-9 but also of caspase-8 and blocked the induction of apoptosis. Thus, we provide evidence that caspase-3 activity is dispensable for MMC-induced apoptosis and for caspase-8 and -9 processing in MCF-7 cells.