Background and purpose We demonstrated previously that the administration of tumor necrosis factor alpha (TNF-α) for the treatment of solid tumors enhanced the response to chemotherapy by augmenting intratumoral drug accumulation. TNF-α changes the integrity of the endothelial cell monolayer in combination with interferon gamma (IFN-γ), which is further enhanced by the addition of peripheral blood mononuclear cells (PBMCs). The improved effect of PBMCs was mostly induced by the endogenous production of interleukin-1beta (IL-1ß) after TNF-α stimulation. In the current study, we demonstrate that exposing endothelial cells to TNF-α and PBMCs mediates the loss of vascular endothelial (VE)-cadherin, an important adherens junction protein for maintaining endothelial integrity, through endogenous IL-1ß. This loss increases permeability of the endothelial layer, thereby explaining the augmented passage of chemotherapeutics into the tumor. Methods Human umbilical vein endothelial cells were exposed to TNF-α, IFN-γ, PBMCs, or IL-1ß, and the effects on the endothelial integrity were assessed by morphological changes and permeability changes with the use of fluorescein isothiocyanate-labeled bovine serum albumin flux. The loss of VE-cadherin was assessed using immunofluorescence, western blotting, and polymerase chain reaction. Results Incubating endothelial cells with TNF-α, IFN-γ, and PBMCs increased cell elongation, gap formation, and subsequently the permeability of fluorescein isothiocyanate-labeled bovine serum albumin compared with control or TNF-α and IFN-γ-treated cells (P <.05). When PBMCs were replaced with IL-1ß, identical changes were observed. These changes in integrity were associated with a loss of VE-cadherin at the membrane. Conclusion We conclude that VE-cadherin is lost at the membrane when endothelial cells are exposed to TNF-α, IFN-γ, and PBMCs, which results in loss of integrity. IL-1ß can mimic the effects of PBMCs, indicating a dominant role of endogenously produced IL-1ß in this process.