Superoxide generation by phagocytic cells requires assembly of the enzymatic complex NADPH oxidase, consisting of membranous and cytosolic components which translocate to the plasma membrane upon cell activation. Two highly homologous cytosolic small GTP-binding proteins, rac1 and rac2, have recently been implicated in the activation of NADPH oxidase. We report that, in resting neutrophils, the amount of rac detectable in the plasma membrane-enriched fraction accounted for 1.5% of the cytosolic protein. When the O2- generation was induced by stimulating neutrophils with PMA, fMLP or opsonized zymosan rac proteins were not recruited at the plasma membrane as analysed by immunoblot or immunofluorescence assays. We conclude that rac proteins do not assemble with the NADPH oxidase complex in the plasma membrane, and we propose that they might instead transduce translocation signals to the other cytosolic components.