Phagocytic cells are characterized by their ability to generate superoxide anions upon activation by appropriate stimuli. UM384, a myelomonocytic cell line, was shown to be defective in this oxidase activity as measured by nitroblue tetrazolium or cytochrome c reduction. Cytochrome b558, a unique pigment present in phagocytes and implicated in electron transfer from NADPH to O2, was absent in the differentiated UM384 cells. Both subunits of the cytochrome b558 appeared to be absent or present in strongly reduced amounts compared to the mother cell line U937, as indicated by immunocytochemistry or Western blot analysis using monoclonal antibodies (MABs). On the other hand, cytosolic factors also involved in NADPH oxidase activity were shown to be present, either immunologically or by using the capacity of the cytosol to activate the oxidase in a membrane fraction from bovine neutrophils. At the molecular level, the mRNA that encodes the gp91-phox was shown to be absent in the differentiated UM384 cells, whereas the mRNA that encodes the p22-phox was normally expressed. These results suggest that the defect in superoxide production by the UM384 cells is related to the absence of cytochrome b558, a situation mimicking that observed in phagocytes from patients with X-linked chronic granulomatous disease (X-CGD).