Chronic granulomatous disease is an X-linked defect in the killing of certain bacteria by peripheral blood granulocytes and may be detected with the nitroblue tetrazolium (NBT) test. The rate of reduction of NBT by normal leukocytes is stimulated by phagocytosis. It also depends on cell number, pH and temperature. Granulocytes of affected male patients failed to reduce NBT to blue formazan during phagocytosis whereas leukocytes of carrier females, usually asymptomatic, demonstrated intermediate dye reduction. A 17year-old girl, who had lifelong recurrent suppurative infections, was identified as having chronic granulomatous disease by the NBT test. Her leukocytes and those of affected males had absent dye reduction, low values for oxygen consumption and diminished NADH oxidase activities during phagocytosis. Her parents' leukocytes had normal dye reduction rather than the intermediate values noted in mothers of affected males. Thus, the disease may also be transmitted as an autosomal recessive trait.