During gram-negative sepsis, human monocytes are triggered to produce large quantities of proinflammatory cytokines such as tumor necrosis factor alpha (TNF-α) in response to endotoxin (lipopolysaccharide [LPS]). Several studies have identified signal transduction pathways that are activated by LPS, including activation of nuclear factor-κB (NF-κB) and activation of mitogen-activated protein kinases (MAPKs), including ERK1 and ERK2, c-Jun N-terminal kinase, and p38. In this study, the relevance of ERK1 and ERK2 activation for LPS-induced TNF-α production by primary human monocytes has been addressed with PD-098059, which specifically blocks activation of MAPK kinase (MEK) by Raf-1. TNF-α levels in the monocyte culture supernatant, induced by 10 ng of LPS/ml, were reduced by PD-098059 (50 μM). In addition, PD-098059 also reduced TNF-α mRNA expression when cells were stimulated for 1 h with LPS. On the other hand, LPS-induced interleukin-10 (IL-10) levels in the monocyte supernatant were only slightly inhibited by PD-098059. Ro 09-2210, a recently identified MEK inhibitor, completely abrogated TNF-α levels at nanomolar concentrations. IL-10 levels also were strongly reduced. To show the efficacy of PD-098059 and Ro 09-2210, ERK1 and -2 activation was monitored by Western blotting with an antiserum that recognizes the phosphorylated (i.e., activated) forms of ERK1 and ERK2. Addition of LPS to human monocytes resulted in activation of both ERK1 and ERK2 in a time- and concentration (50% effective concentration between 1 and 10 ng of LPS/ml)-dependent manner. Activation of ERK2 was blocked by PD-098059 (50 μM), whereas ERK1 seemed to be less affected. Ro 09-2210 completely prevented LPS-induced ERK1 and ERK2 activation. LPS-induced p38 activation also was prevented by Ro 09-2210. These data further support the view that the ERK signal transduction pathway is causally involved in the synthesis of TNF-α by human monocytes stimulated with LPS.