PROFESSIONAL phagocytes, such as neutrophils and monocytes, have an NADPH oxidase that generates superoxide and other reduced oxygen species important in killing microorganisms (reviewed in ref. 1). Several components of the oxidase complex have been identified as targets of genetic defects causing chronic granulomatous disease2-4. The complex consists of an electron transport chain that has as its substrate cytosolic NADPH and which discharges superoxide into the cavity of the intracellular phagocytic vacuole. The only electron transport component identified so far is a low-potential cytochrome b (refs 5,6), apparently the only membrane component required7. At least three cytosolic factors are also necessary, two of which, p67phox and p47phox, have been identified by their absence in patients with chronic granulomatous disease8-11. A third component, ω1 (refs 12, 13), is required for stimulation of oxidase activity in a cell-free system14-16. The active components of purified ω1 are two proteins that associate as heterodimers17, and here we report that these are the small GTP-binding protein p21racl and the GDP-dissociation inhibitor rhoGDI.