Bacterial lipopolysaccharide (LPS) is a powerful activator of the innate immune system. Exposure to LPS induces an inflammatory reaction in the lung mediated primarily by human blood monocytes and alveolar macrophages which release an array of inflammatory chemokines and cytokines including IL-8, TNF-, IL-1 and IL-6. The signaling mechanisms utilized by LPS to stimulate the release of cytokines and chemokines are still incompletely understood. Pre-treatment with the protein tyrosine kinase-specific inhibitors genistein and herbimycin A effectively blocked LPS-induced NF-B activation as well as IL-8 gene expression in human peripheral blood monocytes. However, when genistein was added 2 min after the addition of LPS, no inhibition was observed. Utilizing co-immunoprecipitation assay, we further showed that LPS-stimulated tyrosine phosphorylation of Toll-like receptor4 (TLR4) may be involved in downstream signaling events induced by LPS. These findings provide evidence that LPS-induced NF-B activation and IL-8 gene expression utilize a signaling pathway requiring protein tyrosine kinase and that such regulation may occur through tyrosine phosphorylation of TLR4.