Bacterial LPS triggers monocytes and macrophages to produce several inflammatory cytokines and mediators. However, once exposed to LPS, they become hyporesponsive to a subsequent endotoxin challenge. This phenomenon is defined as LPS desensitization or tolerance. Previous studies have identified some components of the biochemical pathways involved in negative modulation of LPS responses. In particular, it has been shown that the IL-1R-related protein ST2 could be implicated in LPS tolerance. The natural ligand of ST2 was recently identified as IL-33, a new member of the IL-1 family. In this study, we investigated whether IL-33 triggering of ST2 was able to induce LPS desensitization of mouse macrophages. We found that IL-33 actually enhances the LPS response of macrophages and does not induce LPS desensitization. We demonstrate that this IL-33 enhancing effect of LPS response is mediated by the ST2 receptor because it is not found in ST2 knockout mice. The biochemical consequences of IL-33 pretreatment of mouse macrophages were investigated. Our results show that IL-33 increases the expression of the LPS receptor components MD2 (myeloid differentiation protein 2) and TLR-4, the soluble form of CD14 and the MyD88 adaptor molecule. In addition, IL-33 pretreatment of macrophages enhances the cytokine response to TLR-2 but not to TLR-3 ligands. Thus, IL-33 treatment preferentially affects the MyD88-dependent pathway activated by the TLR.