N eutrophil NADPH oxidase plays a key role in host defense and in inflammation by releasing large amounts of superoxide and other ROSs. Proinflammatory cytokines such as GM-CSF and TNF-α prime ROS production by neutrophils through unknown mechanisms. Here we used peptide sequencing by tandem mass spectrometry to show that GM-CSF and TNF-α induce phosphorylation of Ser345 on p47^phox, a cytosolic component of NADPH oxidase, in human neutrophils. As Ser345 is located in the MAPK consensus sequence, we tested the effects of MAPK inhibitors. Inhibitors of the ERK1/2 pathway abrogated GM-CSF–induced phosphorylation of Ser345, while p38 MAPK inhibitor abrogated TNF-α–induced phosphorylation of Ser345. Transfection of HL-60 cells with a mutated p47^phox (S345A) inhibited GM-CSF– and TNF-α–induced priming of ROS production. This event was also inhibited in neutrophils by a cell-permeable peptide containing a TAT-p47^phox-Ser345 sequence. Furthermore, ROS generation, p47^phox-Ser345 phosphorylation, and ERK1/2 and p38 MAPK phosphorylation were increased in synovial neutrophils from rheumatoid arthritis (RA) patients, and TAT-Ser345 peptide inhibited ROS production by these primed neutrophils. This study therefore identifies convergent MAPK pathways on Ser345 that are involved in GM-CSF– and TNF-α–induced priming of neutrophils and are activated in RA. Inhibition of the point of convergence of these pathways might serve as a novel antiinflammatory strategy.