By photoaffinity labeling with a tritiated azido derivative of phenylarsine oxide (PAO), 4[N-(4-azido-2-nitrophenyl)amino-[H-3]acetamido]phenylarsine oxide ([H-3]azidoPAO), we demonstrate that PAO binds selectively to the S100 A8/A9 complex of bovine neutrophil cytosol (previously known as p7/p23, homologous to the MRP-8/MRP-14 complex of human phagocytes). Using a semirecombinant cell free assay of oxidase activation and the determination of oxidase activity by the production of the superoxide anion O-2(-), we found that the PAO binding protein (p7/p23) was able to potentiate the activation of NADH oxidase and that this effect was synergized by PAO. The p7/p23 protein complex of bovine neutrophils can therefore be considered as a positive regulator of NADPH oxidase activation in neutrophils.