Stimulation of human neutrophils by LPS is central to the pathogenesis of sepsis and the adult respiratory distress syndrome. The intracellular signaling pathway that results in cellular responses following LPS stimulation in neutrophils is unknown. We report that exposure of neutrophils to LPS results in the phosphorylation and activation of a p38 mitogen-activated protein (MAP) kinase, occurring in a concentration-dependent manner, with maximum response at 20 to 25 min. Partial purification of a p38 MAP kinase by ion exchange chromatography established it as distinct from the p42/p44 (extracellular signal-regulated kinases (ERK-1 and ERK-2) MAP kinases). Activation of the p38 MAP kinase by LPS in human neutrophils occurs via CD14, a proposed LPS receptor, and requires the presence of plasma containing the LPS-binding protein. This intracellular signaling pathway is independent of protein kinase C and does not involve Raf, MAP/ERK kinase kinase-1, MAP/ERK kinase-1, or MAP/ERK kinase-2 and does not result in the activation of the p42/p44 ERK MAP kinases or the c-jun N-terminal kinases.