The NADPH oxidase is an electron transport chain found in lymphocytes and in the wall of the endocytic vacuole of ‘professional’phagocytic cells. It is so called because NADPH is used as an electron donor to reduce oxygen to superoxide and hydrogen peroxide. The redox components are provided by a very unusual flavocytochrome b from the membrane, which is dependent upon cytosolic factors (including two specialized proteins, p47 phox and p67 phox) for activation. The small GTP-binding protein, p21 rac, is also implicated in this system, possibly as the switch that triggers electron transport. This system provides a key to our understanding of the way in which these GTP-binding proteins function.