The leukocytes of patients with chronic granulomatous disease (CGD) may be identified by their failure to reduce Nitro Blue Tetrazolium (NBT) during phagocytosis. This reaction, normally detected in the phagocytic vacuole, is absent or delayed in CGD monocytes and eosinophils as well as in neutrophils, even though sonicates of normal and CGD leukocytes contain equal activities of a cyanide insensitive enzyme system capable of reduction of NBT in the presence of pyridine nucleotide. Enlargement of CGD phagocytic vacuoles appears to be inhibited. Histochemical estimates of the rate of release of alkaline phosphatase are normal in CGD cells. Peroxidase activity is released from CGD cells, but the rate appears to be somewhat slower than normal in some cases. The latter observation may be explained by the increased intensity of the peroxidase stain in resting and phagocytizing CGD cells. The severity of the defect in NBT reduction within the phagocytic vacuoles of the leukocytes of patients and carriers is more variable than was previously appreciated. Some female carriers have profoundly reduced dye reduction and others are nearly indistinguishable from normal. Three brothers with CGD demonstrated significant, albeit delayed, NBT reduction in phagocytic vacuoles during prolonged incubation of their leukocytes. No obvious relationship exists, however, between the rate of reduction of NBT in vacuoles and the clinical severity of the disease.