Under hypoxia, some cells survive, whereas others are irreversibly injured and die. The factors that determine cell fate under stress remain largely unknown. We have recently selected death-resistant cells via repeated episodes of hypoxia. This study has isolated 80 clones from the selected cells and has characterized their response to apoptotic injury. Compared with the wild-type cells, the isolated clones showed a general resistance to apoptosis, with 13 extremely resistant to azide-induced apoptosis, 10 to staurosporine, and 9 to cisplatin. The cell clones that most consistently demonstrated resistance or sensitivity to injury were further studied for their response to azide treatment. Azide induced comparable ATP-depletion in these clones and wild-type cells. Hypoxia-inducible factor-1 (HIF-1) was up-regulated in several clones, but the up-regulation did not correlate with cell death resistance. The selected clones maintained an epithelial phenotype, showing typical epithelial morphology, forming "domes" at high density, and expressing E-cadherin. Azide-induced Bax translocation and cytochrome c release, two critical mitochondrial events of apoptosis, were abrogated in death-resistant clones. In addition, cell lysates isolated from these clones showed lower caspase activation upon the addition of exogenous cytochrome c. These clones had similar levels of Bax, Bak and Bid as wild-type cells, whereas Bcl-2 expression was higher in all selected clones, and interestingly, Bcl-xL was markedly up-regulated in the most death-resistant clones. The results suggest that apoptotic resistance of the selected clones is not determined by a single factor or molecule, rather by various alterations at the core apoptotic pathway.