We tried to purify a new protein component required for the activation of NADPH oxidase from the guinea pig neutrophil cytosolic fraction which did not contain p47phox and p67phox, using HAC-5CP, IEC-QA and Superose 12HR columns. The NADPH oxidase-activating activity was separated into three fractions on IEC-QA anion-exchange HPLC. However, when each of the fractions was purified by Superose 12HR gel filtration, the active fraction eluted at the same position, and was found to contain a common protein with a molecular weight of 28.5 kDa on SDS-PAGE. These results suggest that the 28.5 kDa protein is a novel NADPH oxidase activating protein.