The NADPH oxidase in neutrophils was specifically solubilized from membrane vesicles of porcine blood neutrophils and rapidly concentrated by immunoprecipitation with cross-reacting anti-P-450 reductase IgG. The precipitates from both myristic acid-stimulated and resting cells contained one third of the cytochrome b-558 and were slightly contaminated with myeloperoxidase. The immunoprecipitate from stimulated cells gave rhombic high-spin ESR signals of a heme at g = 6.47 and 5.49, which were insensitive to KCN, whereas the preparation from resting cells did not give these signals. The rhombic high-spin signals are discussed in view of the participation of cytochrome b-558 in the NADPH oxidase system.