Marburg virus (MARV) infection often causes fulminant shock due to pathologic immune responses and alterations of the vascular system. Cytokines released from virus-infected monocytes/macrophages provoke endothelial activation and vascular hyperpermeability and contribute to the development of shock. Tyrosine phosphorylation of cell-junction proteins is important for the regulation of paraendothelial barrier function. We showed that mediators released from MARV-infected monocytes/macrophages, as well as recombinant tumor necrosis factor (TNF)- alpha /H2O2 and interferon (IFN)- gamma , caused tyrosine phosphorylation of platelet endothelial cell adhesion molecule-1 (PECAM-1) but not of the vascular endothelial (VE) cadherin/catenin complex proteins. Tyrosine phosphorylation of PECAM-1 was associated with delayed opening of interendothelial junctions. Interestingly, we observed an early increase in water permeability in response to TNF- alpha /H2O2 that was not due to an opening of the interendothelial junctions. These data indicate 2 distinct mechanisms for the TNF- alpha /H2O2-mediated decrease in endothelial barrier function involving tyrosine phosphorylation of PECAM-1 but not requiring tyrosine phosphorylation of VE-cadherin or catenin proteins.