LPS binding to Toll-like receptor 4 induces a large number of genes through activation of NF-kappaB and IFN-regulatory factor-3 (IRF-3). However, no previous reports have tested the role of serum proteins in LPS-induced gene expression profiles. To investigate how serum proteins affect LPS-induced signaling, we investigated LPS-inducible genes in PBMC using an oligonucleotide probe-array system. Approximately 120 genes up-regulated by LPS were hierarchically divided into two clusters. Induction of one cluster, containing only IFN-inducible genes, was serum dependent. Real-time PCR analysis confirmed that IFN-inducible genes were induced only in the presence of serum, whereas inflammatory genes were induced both in the presence and absence of serum. Further analysis demonstrated that addition of LPS-binding protein (LBP), but not of soluble CD14 to the serum-free medium enabled the induction of IFN-inducible genes and IFN-beta itself by LPS in human monocytes. The mRNAs for IFN-beta and IFN-inducible genes were induced by LPS only in the presence of serum from LBP(+/+) mice, and not in the presence of serum from LBP(-/-) mice. Blocking experiments also confirmed the involvement of LBP in this phenomenon. Immunoblotting analysis showed that phosphorylation of c-Jun N-terminal kinase, p38, IRF-3, tyrosine kinase 2, and STAT1 by LPS, but not of NF-kappaB and extracellular signal-regulated kinase was abrogated in the absence of LBP. This critical role for LBP implies the presence of possible mechanisms linking LBP to the intracellular signaling between Toll-like receptor 4 and IRF-3, leading to the induction of IFN-beta by LPS.