Structural, spectroscopic and chemical features of the metal centres of non-haem-iron-containing cytochrome b557 are described. This haemoprotein is a member of the ferritin family of proteins and thus is also called bacterioferritin. It consists of 24 polypeptide subunits and, in addition to its inter-subunit bis-methionine coordinated haem b groups, it also contains intra-subunit dinuclear metal centres and has the capacity for a central non-haem-iron deposit of 4,500 Fe(III) ions per molecule. The dinuclear site can be occupied by Co(II) or Mn(II), and probably by Zn(II) and Fe(II) as well. Mixed metal centres also appear to be formed. The non-haem-iron core can be either amorphous, as customarily found with native proteins, or crystalline. Crystalline cores can be laid down in vitro. This latter type of core is often superparamagnetic and this phenomenon is decribed, particularly with regards to its characterisation by 57Fe Mössbauer spectroscopy, EPR spectroscopy and Magnetic circular dichroism spectroscopy. Finally, possible functional roles for the metal centres are described.