Activation of extracellular-regulated kinases 1/2 (ERK) is involved in lipopolysaccharide (LPS)-induced cellular responses such as the increased production of proinflammatory cytokines. However, mitogen-activated protein kinases (MAPKs) such as p38 are also activated by LPS and have been postulated to be important in the control of these end points. Therefore, establishing the relative contribution of MAPKs in each cell type is important, as is elucidating the molecular mechanisms by which these MAPKs are activated in LPS-induced signaling cascades. We demonstrated in DC2.4 dendritic cells that ERK regulates tyrosine phosphorylation of phosphatidyl-inositol-3-kinase (PI3-K) and the production of TNF-alpha. We also demonstrated that Raf1 is phosphorylated and involved in the production of TNF-alpha and tyrosine phosphorylation of PI3-K via ERK. Raf1 also regulates the activation of NF-kappaB. We propose that Raf1 plays a pivotal role in LPS-induced activation of the dendritic cells.