Staurosporine (STS) induces apoptosis in various cell lines. We report in this study that primary cultured mouse hepatocytes are less sensitive to STS compared with Jurkat cells and Huh-7 cells. In contrast to the cell lines, no apparent release of cytochrome c or loss of mitochondrial transmembrane potential was detected in primary hepatocytes undergoing STS-induced apoptosis. Caspase-3 was activated in primary hepatocytes by STS treatment, but caspase-9 and -12 were not activated, and caspase-3 activation is not dependent on caspase-8. These findings point to a novel pathway for caspase-3 activation by STS in primary hepatocytes. Pretreatment with caspase inhibitor converted STS-induced apoptosis of hepatocytes to necrotic cell death without significantly changing total cell death. Thus STS causes hepatocytes to commit to death upstream of the activation of caspases. We also demonstrated that STS dramatically sensitized primary hepatocytes to tumor necrosis factor-alpha-induced apoptosis. STS activated I kappa B kinase and nuclear factor-kappa B (NF-kappa B) nuclear translocation and DNA binding but inhibited transactivation of I kappa B-alpha, inducible nitric oxide synthase, and inhibitor of apoptosis protein-1 in hepatocytes and NF-kappa B reporter in transfected Huh-7 cells.