INT reductase activity in flavocytochromeb₅₅₈ purified from membranes of X-CGD, ▵488-497 gp91^phox, and WT gp91^phoxPLB-985 cells. INT reductase activity was determined in the 96-well microtiter plate assay as described in the Materials and Methods. Each well contained the equivalent of 0.5 pmol flavocytochromeb₅₅₈ and 2.5 × 10⁶ cell equivalents of neutrophil cytosol. The results are expressed as the mean ± SEM.