The NADPH oxidase that produces superoxide in professional phagocytic cells is a flavocytochrome b electron transport chain in the membrane, a heterodimer of gp91^phox and p22^phox, that is activated by a number of cytosolic proteins, including p47^phox p67^phox, and the small GTP-binding protein p21rac, which translocate to the membrane and attach to the flavocytochrome on activation. The components of this oxidase were localized on the cytoplasmic surface of the plasma membrane of adherent unroofed neutrophils by immunolabeling. Components of the NADPH oxidase and p21rac were found together in punctate clusters occupying 0.03–0.1 μm² of the cytoplasmic surface of the plasma membrane where the density of labeling of the cytosolic components was increased after stimulation with phorbol myristate acetate.