Whole-cell lysates from human neutrophils and a transgenic COS-phox cell line were separated by 9% or 12% sodium dodecyl sulfate–polyacrylamide gel electrophoresis. Serial dilutions of human neutrophil lysate (1 μg, 5 μg, 10 μg, and 20 μg protein, corresponding to 3 × 10⁴, 15 × 10⁴, 30 × 10⁴, and 60 × 10⁴ cell equivalents, respectively) were run alongside COS-phox cell lysate (5 μg, 10 μg, and 20 μg protein, corresponding to 1.8 × 10⁴, 3.6 × 10⁴, and 7.2 × 10⁴ cell equivalents, respectively). The blots were probed with primary antibodies against each of the indicated proteins followed by horseradish peroxidase–conjugated secondary antibodies, which were visualized by ECL. Relative expression levels were determined for the COS-phox cells as compared with human polymorphonuclear neutrophils by densitometric analysis of the resultant protein bands. Results were averaged for 2 independent experiments, with the use of neutrophils obtained from 2 healthy human donors. Results for gp91^phox represent fully glycosylated protein; in COS-phox cells, a comparable amount of precursor protein (58 and 65 kd) is also present.