Resting neutrophils possess cytosolic cyanide-sensitive (CNs) superoxide dismutase (SOD) and cyanide-insensitive (CNi) SOD, located in an undefined organelle of the 27,000 g sedimentable fraction of its homogenate. Stimulated neutrophils generate large amounts of superoxide anion, part of which is released in the extracellular medium and contributes to changes that occur in inflammatory foci. Our purpose was to assess whether or not the neutrophil upon stimulation secreted either or both CNs and CNi SOD activity, because the process could protect against the release of superoxide anion. Human neutrophils stimulated in vitro with phorbol myristate acetate released 32.6% and 53% of their content in myeloperoxidase (an azurophilic granule marker) and vitamin B12 binding activity, respectively. The CNi SOD was not secreted at all, whereas 16% and 23% of CNs SOD were released by resting and stimulated neutrophils, respectively. In contrast, lactate dehydrogenase, a cytosolic marker, was released by both resting and stimulated cells (approximately equal to 9%). These results suggest that CNi SOD is not located in the granules but in another organelle that does not degranulate upon stimulation and consequently does not protect against superoxide anion formed by neutrophils in the extracellular medium. In contrast, CNs SOD is slightly but significantly released (P less than .02) and may be protective. Neutrophils from two patients with chronic granulomatous disease behaved similarly to control neutrophils but their content of both types of SOD was higher than that of the controls.