—Tumor-necrosis factor-α (TNF-α) is a proinflammatory cytokine with a wide variety of biological effects. The most important source of this cytokine is monocytes/macrophages. It is a potent agonist in the activation of endothelial cells; however, the precise role of endothelial cells as a source of TNF-α is not known. In the present study, we addressed the possibility that TNF-α is produced by cultured human umbilical vein endothelial cells (HUVEC) stimulated with factors such as lipopolysaccharide (LPS) or interleukin-1α (IL-1α). LPS and IL-1α induced expression of TNF-α mRNA in HUVEC. IL-1α induced expression and secretion of TNF-α protein, but LPS did not induce production of TNF-α protein. Most of the TNF-α protein in cell lysate was found in the membrane fraction. The mRNA for TNF-α–converting enzyme (TACE) was expressed in unstimulated HUVEC, and its level was not altered by treatment with LPS or IL-1α. Transfection of HUVEC with full-length cDNA encoding the precursor TNF-α enhanced secretion of TNF-α protein by these cells, and treatment of the cells with a TACE inhibitor reduced the secretion. These results suggest that HUVEC produce TNF-α and have TACE activity. Secreted TNF-α may be involved in autocrine activation of endothelial cells, and TNF-α retained in cell membrane may serve as a juxtacrine system to activate target cells on the endothelial surface.