The leukocyte NADPH oxidase catalyzes the reduction of oxygen to O(2)(-) at the expense of NADPH. Extensive phosphorylation of the oxidase subunit p47(PHOX) occurs during the activation of the enzyme in intact cells. p47(PHOX) carrying certain serine-to-alanine mutations fails to support NADPH oxidase activity in intact cells, suggesting that the phosphorylation of specific serines on p47(PHOX) is required for the activation of the oxidase. Earlier studies with both intact cells and a kinase-dependent, cell-free system have suggested that protein kinase C can phosphorylate those serines of p47(PHOX) whose phosphorylation is necessary for its activity. Work with inhibitors suggested that a phosphatidylinositol 3-kinase-dependent pathway also can activate the oxidase. Phosphorylation of p47(PHOX) by Akt (protein kinase B), whose activation depends on phosphatidylinositol 3-kinase, could be the final step in such a pathway. We now find that Akt activates the oxidase in vitro by phosphorylating serines S304 and S328 of p47(PHOX). These results suggest that Akt could participate in the activation of the leukocyte NADPH oxidase.