Tpl 2 knockout mice produce low levels of TNF-α when exposed to lipopolysaccharide (LPS) and they are resistant to LPS/D-Galactosamine-induced pathology. LPS stimulation of peritoneal macrophages from these mice did not activate MEK1, ERK1, and ERK2 but did activate JNK, p38 MAPK, and NF-κB. The block in ERK1 and ERK2 activation was causally linked to the defect in TNF-α induction by experiments showing that normal murine macrophages treated with the MEK inhibitor PD98059 exhibit a similar defect. Deletion of the AU-rich motif in the TNF-α mRNA minimized the effect of Tpl2 inactivation on the induction of TNF-α. Subcellular fractionation of LPS-stimulated macrophages revealed that LPS signals transduced by Tpl2 specifically promote the transport of TNF-α mRNA from the nucleus to the cytoplasm.