The goal of this investigation was to determine the effect(s) of the bacterial component, lipopolysaccharide, on leukotriene C4 (LTC4) synthase gene expression in mononuclear phagocytes. Lipopolysaccharide treatment for 7 days resulted in decreased stimulated LTC4 release from the monocyte-like cell line, THP-1. Reverse transcriptase polymerase chain reaction studies demonstrated expression of the putative lipopolysaccharide receptor, Toll-like receptor 4, in both THP-1 cells and the eosinophil-like cell line, AML14.3D10. Conditioning with high-dose lipopolysaccharide resulted in cell-specific downregulation of LTC4 synthase messenger RNA in THP-1 cells, but not in AML14.3D10 cells. Treatment of THP-1 cells with lipopolysaccharide demonstrated that the inhibitory effect is serotype-independent, dose-dependent (at doses from 0.001 to 100 ng/mL), and time-dependent, occurring as early as 4 h after exposure. Actinomycin-D studies indicate that lipopolysaccharide does not accelerate the rate of LTC4 synthase messenger RNA decay. Transient transfection of THP-1 cells with a luciferase reporter construct containing the first 1.35 kilobases of the LTC4 synthase promoter demonstrated that lipopolysaccharide decreases LTC4 synthase promoter activity. Treatment of THP-1 cells with a specific inhibitor of nuclear factor-κB resulted in abrogation of the effect of lipopolysaccharide on LTC4 synthase gene expression. Induced activation of nuclear factor-κB resulted in a time-dependent decrease in LTC4 synthase messenger RNA. Cell-specific lipopolysaccharide modulation of LTC4 synthase gene expression may have important implications for understanding the role of bacterial exposure in the pathogenesis of inflammatory lung diseases, such as asthma.