The bacteriocidal capacity of phagocytic cells is impaired in X-linked chronic granulomatous disease (X-CGD), a disorder characterized by the absence of functional plasma-membrane-associated NADPH oxidase¹. The components of this oxidase system, their correspondence with specific genetic loci, and the primary protein defect in X-CGD remain incompletely defined. We recently reported cloning of the putative X-CGD gene on the basis of DNA linkage^2,3. To identify the predicted protein in vivo, antibodies were raised to a synthetic peptide derived from the complementary DNA sequence and to a fusion protein produced in Escherichia coli. In Western blots antisera detect a neutrophil protein of relative molecular mass in 90,000 (90K) that is absent in X-CGD patients. Antisera also react with the larger component of cytochrome b recently purified from neutrophil plasma membranes as a complex of glycosylated 90K and non-glycosylated 22K polypeptides⁴. Based on our identification of the X-CGD protein in vivo, we propose that one of its critical roles is to interact with the 22K species to form a functional cytochrome b complex.