We have established a monoclonal hybridoma clone that produces IgG1 against the cytochrome b558 of human neutrophils. The antibody 7D5, secreted by the hybridoma, bound to solubilized cytochrome b of the neutrophils but not to other proteins such as hemoglobin, myeloperoxidase, and pig cytochrome P-450. Immunocytochemical studies of normal human peripheral blood showed that 7D5 bound to neutrophils and monocytes but not to lymphocytes or erythrocytes. The neutrophils of male patients but not of a female patient with chronic granulomatous disease lacked the antigen of 7D5 as well as the absorption spectrum for cytochrome b558. A mosaic of the antigen-positive and -negative neutrophils was observed in mothers of the male patients. These biochemical and immunocytochemical results indicate that 7D5 is a specific antibody against cytochrome b558 of human phagocytes.