The expression of class I and class II major histocompatibility complex (MHC) molecules on the surface of cultured human umbilical vein endothelial cells (HUVEC) incubated with alpha tumor necrosis factor (TNF) or interferon-gamma (IFN-gamma) was determined by fluorescence flow cytometry. HUVEC were stained with fluorescein conjugated monoclonal antibodies (MoAbs) directed against monomorphic determinants of class I MHC molecules, HLA-A, B alpha chain (HLA-alpha) and beta-2-microglobulin (B2m), and class II MHC molecules, HLA-DR and HLA-DQ. HUVEC exposed to TNF for 96 hr increase their expression of class I MHC molecules two- to fourfold. Similarly, IFN-gamma increases the expression of class I MHC molecules two- to fivefold. No induction of HLA-DR or HLA-DQ was seen following exposure to TNF. IFN-gamma induces the appearance and markedly increases the expression of HLA-DR following 96 hr incubation. HLA-DQ was also induced by IFN-gamma but to a much lesser extent. TNF in combination with IFN-gamma enhances HUVEC expression of HLA-alpha, B2m, and HLA-DQ greater than that observed with either mediator alone. TNF and IFN-gamma reduced HUVEC expression of HLA-DR below the level observed with IFN-gamma alone. Indomethacin (INDO) does not effect expression of HUVEC class I or class II MHC molecules induced by TNF, IFN-gamma, or the combination of these mediators. These immune mediators produce unique and common effects on HUVEC, and therefore may act by at least two separate mechanisms, independent of the cyclooxygenase pathway, which regulate HUVEC expression of class I and class II MHC surface molecules.