Acetyl-CoA:1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase, along with phospholipase A 2 , is a key regulator of platelet-activating factor biosynthesis via the remodeling pathway. We have now obtained evidence in human neutrophils indicating that this enzyme is regulated by a specific member of the mitogenactivated protein kinases, namely the p38 kinase. We earlier demonstrated that tumor necrosis factor-␣ (TNF-␣) as well as N-formyl-methionyl-leucyl-phenylalanine treatment leads to increased phosphorylation and activation of p38 kinase in human neutrophils. Strikingly, in the present study these stimuli increased the catalytic activity of acetyltransferase up to 3-fold, whereas 4-phorbol 12-myristate 13-acetate, which activates the extracellular-regulated kinases (ERKs) but not p38 kinase, had no effect. Furthermore, a selective inhibitor of p38 kinase, SB 203580, was able to abolish the TNF-␣-and N-formyl-methionyl-leucyl-phenylalanineinduced activation of acetyltransferase. The same effect was not observed in the presence of an inhibitor that blocked ERK activation (PD 98059). Complementing the findings in intact cells, we have shown that recombinant, activated p38 kinase added to microsomes in the presence of Mg 2؉ and ATP increased acetyltransferase activity to the same degree as in microsomes obtained from TNF-␣-stimulated cells. No activation of acetyltransferase occurred upon treatment of microsomes with either recombinant, activated ERK-1 or ERK-2. Finally, the increases in acetyltransferase activity induced by TNF-␣ could be ablated by treating the microsomes with alkaline phosphatase. Thus acetyltransferase appears to be a downstream target for p38 kinase but not ERKs. These data from whole cells as well as cell-free systems fit a model wherein stimulus-induced acetyltransferase activation is mediated by a phosphorylation event catalyzed directly by p38 kinase.