No / 1 In this study we examined the signalling events that regulate lipopolysaccharide (LPS)-stimulated induction of interferon regulatory factor (IRF)-1 in human umbilical vein endothelial cells (HUVECS). 2 LPS stimulated a time- and concentration-dependent increase in IRF-1 protein expression, an effect that was mimicked by the cytokine, tumour necrosis factor (TNF)-¿. 3 LPS stimulated a rapid increase in nuclear factor kappa B (NFKB) DNA-binding activity. Preincubation with the NFKB pathway inhibitors, N-¿-tosyl-L-lysine chloromethyl ketone (TLCK) or pyrrolidine dithiocarbamate (PDTC), or infection with adenovirus encoding IKB¿, blocked both IRF-1 induction and NFKB DNA-binding activity. 4 LPS and TNF¿ also stimulated a rapid activation of gamma interferon activation site/gamma interferon activation factor (GAS/GAF) DNA-binding in HUVECs. Preincubation with the Janus kinase (JAK)-2 inhibitor, AG490 blocked LPS-stimulated IRF-I induction but did not affect GAS/ GAF DNA-binding. 5 Preincubation with TLCK, PDTC or infection with I¿Ba adenovirus abolished LPS-stimulated GAS/GAF DNA-binding. 6 Incubation of nuclear extracts with antibodies to RelA/p50 supershifted GAS/GAF DNA-binding demonstrating the involvement of NF¿B isoforms in the formation of the GAS/GAF complex. 7 These studies show that NF¿B plays an important role in the regulation of IRF-1 induction in HUVECs. This is in part due to the interaction of NF¿B isoforms with the GAS/GAF complex either directly or via an intermediate protein.