Quorum sensing (QS) is a type of cell-to-cell communication. The Pseudomonas aeruginosa QS molecule N-3-(oxododecanoyl)-l-homoserine lactone (3-o-C12-HSL) has the potential to modulate the immune system of its host. However, the mechanism of that activity is yet to be fully characterized. To be able to understand this activity, we determined whether 3-o-C12-HSL has a direct effect on the immune function and the expression of toll-like receptors (TLRs) in monocytes. Monocytes were cultured with 3-o-C12-HSL at different concentrations (0, 10, 25, 50, and 100 mu mol/L) for 12 h; upon exposure to 3-o-C12-HSL, IL-12 production in monocytes was inhibited, monocyte proliferation was blocked, TLR2- and 4-mRNA expressions were reduced, and TLR5-mRNA expression was increased in a dose-dependent manner. Strikingly, 3-o-C12-HSL was able to significantly induce mRNA changes in the monocytes even at the lowest concentration (10 mu mol/L, P < 0.05). Interestingly, though TLR2- and 4-protein levels were reduced, TLR5 protein expression was not changed. These findings provide a new perspective toward understanding the persistence of chronic inflammation in P. aeruginosa infections. They also suggest that TLR2, 4, and 5 may not share the same signaling pathways during monocyte activation.