The role of iron in certain clinical infections is revealed. In normal persons the antibacterial and antifungal properties of blood and other tissue fluids cannot be maintained unless there are exceptionally low levels of available iron. This is controlled by the presence of the unsaturated iron-binding proteins, transferrin and lactoferrin. In several clinical conditions an abnormal availability of iron is responsible for fatal septicaemia. This is because the phagocytic system is overwhelmed by rapidly growing organisms when iron is freely available.