Activation of the extracellular signal-regulated kinase (ERK) pathway has been shown to occur in monocytes following stimulation with LPS. However, the importance of this event for monocyte function is not clear. To address this issue, we used the novel MAP/ERK kinase (MEK) inhibitor, U0126. Stimulation of monocytes with LPS resulted in activation of the mitogen-activated protein kinase (MAPK) family members ERK, Jun NH 2-terminal kinase (JNK), and p38. Treatment of monocytes with LPS in the presence of U0126 blocked the activation of ERK1 and ERK2. However, the activation of Jun NH 2-terminal kinase and p38 family members was not affected by the compound, confirming the selectivity of U0126. To examine the effects of MEK inhibition on monocyte function, we measured production of the cytokines IL-1, IL-8, and TNF, as well as PGE 2. Monocytes treated with LPS in the presence of U0126 failed to release IL-1, IL-8, TNF, or PGE 2. The failure to secrete IL-1 and TNF was due to decreased levels of mRNA. These results demonstrate that activation of MEK/ERK is critical for cytokine and PGE 2 production by monocytes in response to LPS.