Rho and Rac, two members of the Ras superfamily of guanosine triphosphate (GTP)-binding proteins, regulate a variety of signal transduction pathways in eukaryotic cells. Upon stimulation of phagocytic cells, Rac enhances the activity of the enzyme nicotinamide adenine dinucleotide phosphate (reduced) (NADPH) oxidase, resulting in the production of superoxide radicals. Activation of the NADPH oxidase requires the assembly of a multimolecular complex at the plasma membrane consisting of two integral membrane proteins, gp91^phox and p21^phox, and two cytosolic proteins, p67^phox and p47^phox. Rac1 interacted directly with p67^phox in a GTP-dependent manner. Modified forms of Rac with mutations in the effector site did not stimulate oxidase activity or bind to p67^phox. Thus, p67^phox appears to be the Rac effector protein in the NADPH oxidase complex.