Human neutrophils respond with a potent and sustained production of superoxide by NADPH oxidase activity after exposure to various stimulants. Superoxide is then converted to other reactive oxygen species. These oxidants participate in microbial killing, cause injury to surrounding tissues, regulate apoptosis of neutrophils, and, further, are viewed as a signal transduction messenger. We present herein the cytochemical approach for detecting NADPH oxidase activity in human neutrophils under both an electron microscope and a CCD camera-equipped fluorescence microscope. The applications of the cytochemical methodology are also mentioned in this review. Especially, the following mechanisms for the release of superoxide from human neutrophils are considered: the active NADPH oxidase is located exclusively in intracellular compartments which are delivered to the plasma membrane by membrane fusion events, and the vesicles whose membrane contain the active oxidase cycle between the interior of the cell and the plasma membrane.