To maintain the integrity of the vascular barrier, endothelial cells (EC) are resistant to cell death. The molecular basis of this resistance may be explained by the function of antiapoptotic genes such as bcl family members. Overexpression of Bcl-2 or Bcl-X(L) protects EC from tumor necrosis factor (TNF)-mediated apoptosis. In addition, Bcl-2 or Bcl-X(L) inhibits activation of NF-κB and thus upregulation of proinflammatory genes. Bcl-2- mediated inhibition of NF-κB in EC occurs upstream of IκBα degradation without affecting p65-mediated transactivation. Overexpression of bcl genes in EC does not affect other transcription factors. Using deletion mutants of Bcl-2, the NF-κB inhibitory function of Bcl-2 was mapped to bcl homology domains BH2 and BH4, whereas all BH domains were required for the antiapoptotic function. These data suggest that Bcl-2 and Bcl-X(L) belong to a cytoprotective response that counteracts proapoptotic and proinflammatory insults and restores the physiological anti-inflammatory phenotype to the EC. By inhibiting NF-κB without sensitizing the cells (as with IκBα) to TNF- mediated apoptosis, Bcl-2 and Bcl-X(L) are prime candidates for genetic engineering of EC in pathological conditions where EC loss and unfettered activation are undesirable.