Previous studies have established that different cytokines induce distinct patterns of activation in cultured endothelial cells (EC). Treatment of EC with either TNF or IL-1 causes transient induction of endothelial leukocyte adhesion molecule-1 (ELAM-1) and a sustained increase in intercellular adhesion molecule-1 (ICAM-1) expression. TNF but not IL-1 also increases class I MHC Ag expression. IFN-gamma, which by itself increases EC class I MHC and ICAM-1 but does not induce ELAM-1 expression, has been found to act synergistically with TNF to increase class I expression. In our study, we have further examined IFN-gamma effects on both TNF and IL-1 beta responses. In contrast to IFN-gamma plus TNF cotreatment, IFN-gamma up-regulation of class I MHC molecules is not augmented by cotreatment with IL-1 beta. IFN-gamma plus TNF cotreatment synergistically increases ICAM-1 expression by 24 h of cotreatment, whereas IFN-gamma plus IL-1 beta cotreated EC show at most additive increases. IFN-gamma increases TNF-induced ELAM-1 expression such that a greater number of EC express ELAM-1 at both 4 and 24 h in the presence of IFN-gamma plus TNF compared to cultures treated with TNF alone, although the maximal level of surface expression on individual cells is not increased. Inasmuch as these times represent pre- and post-peak expression time (6 h), respectively, IFN-gamma appears both to accelerate and to prolong transient ELAM-1 expression. Although similar interactions are seen with IL-1 beta, IFN-gamma has a consistently greater effect on TNF-induced compared to IL-1-induced ELAM-1 expression. We next explored the possible mechanism(s) of the synergy between IFN-gamma and TNF. IFN-gamma and TNF do not cooperatively enhance total protein synthesis. Unexpectedly, IFN-gamma and IL-1 beta combine to depress protein synthesis, which may contribute to the failure of these cytokines to positively interact. IFN-gamma and TNF cooperatively increase ELAM-1 mRNA at 6 h and ICAM-1 mRNA at both 6 and 24 h, whereas IFN-gamma and IL-1 show markedly less cooperative augmentation of these transcripts. We conclude that IFN-gamma enhances TNF-induced EC activation by selectively and synergistically increasing synthesis of specific surface molecules, whereas IL-1-induced EC activation is largely unaffected by IFN-gamma.