Lipopolysaccharide (LPS) and interferon (IFN)-γ synergistically induced interleukin-8 (IL-8) production in human monocytic THP-1 cells. IFN-γ-primed THP-1 cells produced higher levels of IL-8 on stimulation with LPS than non-primed cells and the level correlated with duration of priming up to 24 h, although the level of IL-8 induced was most comparable to that induced by costimulation with LPS and IFN-γ. Unstimulated THP-1 cells were shown by flow cytometry to be practically devoid of membrane CD14 (mCD14). LPS and IFN-γ enhanced mCD14 and Toll-like receptor (TLR) 4 expression in THP-1 cells, respectively, and co-stimulation with LPS and IFN-γ induced higher levels of mCD14 and TLR4 expression than stimulation with either agent alone. LPS and IFN-γ alone each augmented MD-2 and MyD88 mRNA expression in THP-1 cells, and costimulation with LPS and IFN-γ markedly enhanced MD-2 and MyD88 mRNA expression in the cells compared to those with either LPS or IFN-γ alone. Anti-CD14 and anti-TLR4 monoclonal antibodies almost completely inhibited IL-8 production induced by LPS plus IFN-γ in THP-1 cells. These findings suggest that combined stimulation of THP-1 cells with LPS and IFN-γ up-regulate mCD14, TLR4, MD-2 and MyD88 expression by these cells, which might be involved in synergistic IL-8 production by the cells.