The iron storage protein ferritin is found in all cells of the body as multiple isoferritins composed of 24 sub units of two types. The structure is well understood from increasingly detailed analysis by X-ray crystallography. Genes for the principal subunits (called H and L) have been cloned and are located on chromosomes 11q and 19q respectively. The production of H24 and L24 recombinant molecules is making it possible to explore the relationship between structure and function. The control of ferritin synthesis by iron at the level of translation is providing a model for understanding the control of protein synthesis. H-rich isoferritins are of considerable interest in haematology as they appear to be implicated in control of haemopoiesis and development of malignancy. Whether or not an abnormality in ferritin is the cause of hereditary haemochromatosis is not yet resolved.