Calpains and caspases are two cysteine protease families that play important roles in regulating pathological cell death. Here, we report that m-calpain may be responsible for cleaving procaspase-12, a caspase localized in the ER, to generate active caspase-12. In addition, calpain may be responsible for cleaving the loop region in Bcl-xL and, therefore, turning an antiapoptotic molecule into a proapoptotic molecule. We propose that disturbance to intracellular calcium storage as a result of ischemic injury or amyloid β peptide cytotoxicity may induce apoptosis through calpain- mediated caspase-12 activation and Bcl-xL inactivation. These data suggest a novel apoptotic pathway involving calcium-mediated calpain activation and cross-talks between calpain and caspase families.