Rabbit polymorphonuclear leucocytes contain the iron-binding protein lactoferrin, and can rapidly phagocytose and destroy Pseudomonas aeruginosa. The lactoferrin normally has a large unsaturated iron-binding capacity. If the cells are exposed to a ferritin-antibody complex, large amounts of this are phagocytosed and appear in the cytoplasmic granules and phagosomes. This leads to saturation of the cellular iron-binding protein with Fe. In these circumstances, the bactericidal power of the cells is greatly reduced with the result that some phagocytosed bacteria survive and eventually grow and destroy the cells. An apoferritin-antibody complex used as a control is also phagocytosed but has no effect on the bactericidal power of the cell. The results support the view that lactoferrin plays an essential role in the bactericidal power of the cell.