The heme-containing protein cytochrome b_-245 has been proposed as a primary component of the microbicidal oxidase system of phagocytes that normally generates superoxide-free radicals but when defective is associated with chronic granulomatous disease. We measured this cytochrome in granulocytes from 27 patients with chronic granulomatous disease and from 64 members of their families. It was undetectable in all 19 of the men in whom the defect appeared to be located on the X chromosome. Female relatives who were heterozygous carriers had reduced concentrations of the cytochrome and variable proportions of cells that were unable to generate Superoxide; these two characteristics were closely related (r = 0.93 in the 16 mothers and 0.85 in all 24 carriers, P<0.001). In contrast, in all eight patients (seven women) with a probable autosomal recessive inheritance the cytochrome was present but nonfunctional. The properties tested, including midpoint potential, carbon monoxide binding, and organelle distribution, were normal, but the cytochrome did not undergo reduction on cellular stimulation. Thus, absence or malfunction of the cytochrome b_-245 may be the causal molecular defect in chronic granulomatous disease, implicating it in the microbicidal oxidase system. (N Engl J Med. 1983; 308:245–51.)