Macrophage activation by bacterial lipopolysaccharide (LPS) promotes the secretion of pro‐inflammatory cytokines, such as tumor necrosis factor‐α (TNF‐α) and interleukin‐1β (IL‐1β), and of secondary mediators, such as leukotrienes and prostaglandins (PGs). Mice lacking the gene encoding the serine/threonine protein kinase Tpl2/Cot produce low levels of TNF‐α in response to LPS because of an ERK‐dependent post‐transcriptional defect, and they are resistant to LPS/d‐galactosamine‐induced endotoxin shock. In this study we demonstrate that prostaglandin E2 and its regulatory enzyme, COX‐2, are also targets of Tpl2‐transduced LPS signals in bone marrow‐derived mouse macrophages. Thus, LPS‐stimulated Tpl2−/− macrophages express low levels of COX‐2 and PGE2, compared with wild‐type Tpl2+/+ cells. The ability of Tpl2 to regulate COX‐2 expression depends on ERK signals that activate p90Rsk and Msk1, which in turn phosphorylate CREB, a key regulator of COX‐2 transcription. These data identify physiological targets of Tpl2 signaling downstream of ERK and further implicate Tpl2 in the pathophysiology of inflammation.