We investigated the ability of TNF-alpha to mediate damage of endothelial cells in the presence of neutrophils, by measuring detachment of cultured human umbilical vein endothelial cells (HUVEC). Endothelial cell detachment was increased from 5% to about 75% by the presence of 1-10 ng/ml TNF-alpha during incubation with neutrophils, whereas negligible endothelial cell lysis was observed as measured by 51Cr release. TNF-alpha was compared with the cytokines IL-1 alpha and IFN-gamma and with PMA and LPS. Both TNF-alpha and PMA appeared to be strong triggers for neutrophil-induced endothelial cell detachment, whilst reduced injury was seen after addition of IL-1 alpha and LPS. IFN-gamma did not induce endothelial cell detachment, but potentiated the effect of both TNF-alpha and IL-1 alpha. TNF-alpha-induced endothelial cell detachment was neutrophil dependent, since pre-incubation of neutrophils, but not pre-incubation of endothelial cells with TNF-alpha, caused endothelial cell detachment. Thus, TNF-alpha-induced increase in neutrophil-adhesiveness of HUVEC was found not to be essential for endothelial damage. Pre-incubation of neutrophils in suspension with TNF-alpha induced rapid activation, followed by nearly complete deactivation of neutrophils, as measured by their capacity to induce detachment of endothelial cells after removal of TNF-alpha. These results indicate that local presence of TNF-alpha might be critical in tissue or organ damage during early, neutrophil-mediated inflammatory processes, independent of enhanced adhesiveness of endothelium for neutrophils.