The iron storage proteins, ferritins, are found in all organisms which use iron. Here iron storage processes in the Escherichia coli ferritin (EcFtnA) are compared with those in human H-type ferritin (HuHF). Both proteins contain dinuclear iron centres that enable the rapid oxidation of 2 Fe(II) by O2. The presence of a third iron binding site in EcFtnA, although not essential for fast oxidation, causes the O2/Fe ratio to increase from 2 to 3-4. In EcFtnA the rate of iron oxidation falls markedly after the oxidation of 48 Fe(II) atoms/molecule probably because some of it remains at the oxidation site. However a compensatory physiological advantage is conferred because this iron is more readily available to meet the cell's needs.