Neutrophils represent the first line of defence against invading microorganisms. An important part of this defence mechanism is the generation of superoxide (O2*-) and its reactive derivatives after stimulation by a variety of agents. This oxidant production is linked to the activation of NADPH oxidase, which is composed of cytosolic components (p47-phox and p67-phox) and membrane components (p22-phox and gp91-phox). Previous studies have shown that NADPH oxidase resides in the plasma membrane and the traditional view holds that cytoplasmic components of NADPH oxidase are brought into the neighbourhood of the plasma membrane and then conjugated with its membrane components upon stimulation. This review focuses on the evaluation of NADPH oxidase-activated sites in human neutrophils. Based on electron microscopic analysis, O2*- is first generated upon stimulation with certain stimulants, such as phorbol myristate acetate, within a specialized intracellular compartment containing alkaline phosphatase, and not on the plasma membrane, as previously thought. In addition, the cytosolic component of NADPH oxidase, p47-phox, accumulates at the juxtaposition of intracellular compartments but not of the plasma membrane. These results demonstrate that initial O2*- production occurs in an intracellular pool in human neutrophils. The oxidant-producing granules then bind directly to the plasma membrane or fuse to form larger structures that eventually become to be associated with the plasma membrane, and O2*- is released extracellularly from the neutrophils.