Recombinant human lymphotoxin (LT) was compared with recombinant human tumor necrosis factor (TNF) for direct actions on cultured human endothelial cells (HEC). At equivalent half-maximal concentrations (based on L929 cytotoxicity units) LT and TNF each caused rapid and transient induction (peak 4 to 6 hr) of an antigen associated with leukocyte adhesion (detected by monoclonal antibody H4/18), a rapid but sustained increased expression (plateau 24 hr) of a lymphocyte adhesion structure (ICAM-1), a gradual (plateau 4 to 6 days) increase in expression of HLA-A,B antigens, and gradual (4 to 6 days) conversion of HEC culture morphology from epithelioid to fibroblastoid, an effect enhanced by immune interferon (IFN-gamma). Induction of H4/18 binding by maximal concentrations of LT or TNF could not be augmented by addition of the other cytokine, and 24 hr pretreatment with LT or TNF produced hyporesponsiveness to both mediators for reinduction. H4/18 binding can be transiently induced by tumor-promoting phorbol esters. Pretreatment with either LT or TNF also fully inhibited induction of H4/18 binding by phorbol ester, whereas phorbol ester pretreatment only variably and partially inhibited reinduction by LT or TNF. These actions of LT on endothelium shared with TNF may serve in vivo to promote lymphocyte and inflammatory leukocyte adhesion and transendothelial migration. Recombinant human interleukin 1 species (IL 1 alpha and IL 1 beta) shared many of the actions of LT and TNF and were indistinguishable from each other. However, IL 1 species could be distinguished from LT/TNF by their relative inability to enhance HLA-A,B expression, by their ability to augment H4/18 binding caused by maximally effective concentrations of LT or TNF, and by their inability to inhibit reinduction of H4/18 binding by LT or TNF. In contrast to the actions of LT or TNF, pretreatment with IL 1 alpha or IL 1 beta only partially inhibited induction of H4/18 binding by phorbol ester, and phorbol ester pretreatment consistently, albeit partially, inhibited induction by IL 1 species. These studies suggest that activated T cells through the secretion of LT can in turn activate the local endothelial lining so as to promote homing and extravasation of inflammatory cells. Furthermore, these LT actions can be augmented or complemented by other locally produced mediators such as IFN-gamma or IL 1.