Chronic granulomatous disease (CGD) is a hereditary illness generally occurring in childhood, in the form of recurrent severe infections. The main pathogens are staphylococci and aspergilli. It results from a failure of professional phagocytes, and particularly neutrophils, to produce superoxide ions O2- and their derivatives, which protect cells from bacterial, invasion through an oxidative and toxic defence mechanism. At an infection site. contact between the neutrophils and microorganisms or an inflammatory mediator triggers a respiratory burst, which results in the activation of the NADPH oxidase enzyme complex. NADPH depletes surrounding oxygen to yield O2-. In its active form. NADPH oxidase is an assembly of two components, namely the membrane cytochrome b558 (consisting o two subunits, gp91-phox and p22-phox) and soluble protein factors present in the resting neutrophil cytoplasm. Transfer of these cytosolic factors and their anchorage to cytochrome b558 determines the activity of NADPH oxidase. The respiratory burst lasts no more than a few minutes, but the precise mechanisms underlying its termination are not well known. In chronic granulomatous disease, neutrophils have lost their bactericidal capacity The most frequent form is hereditary and X-linked; in this case, the affected gene is CYBB, which encodes gp91-phox, the catalytic subunit of cytochrome b558. In autosomal and recessive forms of CGD the mutations affect the genes encoding p22-phox, p67-phox or p47-phox. We have unraveled the assembly mechanisms of the NADPH oxidase complex and have demonstrated that the cytosolic factor p67-phox is the determining element: it triggers both the assembly and the activation of NI4DPH oxidase. Binding of p67-phox to cytochrome b558 induces a gradual conformational change of cytochrome b558, which then becomes capable of transferring electrons produced in the cytoplasm from NADPH to oxygen, reducing the latter to O2-. The isolation of NADPH oxidase in its active and assembled form has allowed us to identify the activation partners of the oxidase complex. We also demonstrated that calcium-binding myeloid-related proteins (MRP). that are abundant in neutrophil cytoplasm, play a fundamental role in this activation. CGD patient management is essentially based on long-term high-dose prophylactic antibiotic administration. Gene therapy is promising but some distance away from practical application. We are currently investigating a new therapeutic concept that consists of transferring cytochrome b558 protein directly into deficient cells (initially the PLB 985 X cell line), encapsulated in proteoliposomes, which are hydrophobic.