Endothelial cells line the vessels and lymphatics of the body, acting as a barrier between the blood and extravascular tissue. These cells are, therefore, in a prime position to play a role in lymphocyte activation. Indeed, it has been shown that primary endothelial cells in culture are capable of presenting particulate and soluble antigens to T cells and that this response is not dependent on macrophages. Recently, we developed an immortalized line of human microvascular endothelial cells, CDC/EU.HMEC-1 (HMEC-1). This endothelial line has the advantage not only of being devoid of contaminating cells but also of being a continuous cell line and therefore not subject to a restricted number of useful passages. The focus of this study was to determine whether HMEC-1 cells (like primary endothelial cells) could present antigen to T cells in the absence of macrophages. We demonstrate that a cloned and purified endothelial cell line can independently provide all the necessary signals for antigen-specific T-cell activation.