Background: Montelukast is a potent cysteinyl leukotriene 1 receptor antagonist possessing some anti-inflammatory effects although the molecular mechanism of these antiinflammatory effects are unknown. In this study, we aimed to investigate the effect of montelukast on NF-kB-associated histone acetylation activity in PMA-differentiated U937 cells. Methods: We examined the inhibitory effects of montelukast on TNF-a-induced IL- 8 production in PMA-differentiated U-937 cells. U-937 cells were exposed to PMA (50 ng/mL) for 48 h to allow differentiation to macrophages. Macrophages were then exposed to TNF-a (10 ng/mL) in the presence or absence of montelukast (0.01– 10 mM) for 24 h. After this time the concentration of IL-8 in the culture supernatant was measured by sandwich-type enzymelinked immunosorbent assay (ELISA) kit. The effect of signalling pathways on TNF-ainduced IL-8 release was examined pharmacologically using selective NF-kB/IKK2 (AS602868, 3 mM), MEK (PD98059, 10 mM) and p38 MAPK (SB203580, 1 mM) inhibitors. NF-kB DNA binding activity was measured by a DNA-binding ELISAbased assay. NF-kB-p65-associated histone acetyltransferase (HAT) activity was measured by immunoprecipitation linked to commercial flourescent HAT. Results: TNFa-induced IL-8 release was suppressed by an NF-kB inhibitor but not by MEK or p38 MAPK inhibitors. Montelukast induced a concentration-dependent inhibition of TNFa-induced IL-8 release and mRNA expression which reached a plateau at 0.1 mM without affecting cell viability. Montelukast did not affect NFkB p65 activation as measured by DNA binding but suppressed NF-kB p65-associated HAT activity. Conclusion: Montelukast inhibits TNFa-stimulated IL-8 expression through changes in NF-kB p65-associated HAT activity. Drugs targetting these enzymes may enhance the anti-inflammatory actions of montelukast