Ferritins are iron-storage proteins capable of holding up to 4500 Fe(3+) ions within a single water-soluble protein shell made from 24 polypeptide chains. The Glu128Arg/Glu135Arg mutants of Escherichia coli and Rhodobacter capsulatus bacterioferritins are unable to associate into 24-meric structures, with dimers of polypeptide chains being their stable forms. The aerobic addition to these of up to 8-10 or 14-20 Fe(2+) ions per dimer, respectively, results in the oxidation of the added Fe(2+) to Fe(3+). Gel permeation chromatography and sedimentation equilibrium studies confirm that the Fe(3+) ions are associated with the polypeptide dimer, and the lack of intense EPR signals from magnetically isolated Fe(3+) ions confirms the formation of one or more antiferromagnetically coupled clusters of Fe(3+) ions. The effect of Fe(3+) chelators on iron-loaded subunit dimers is to remove the Fe(3+) from the protein, but to do so slowly, consistent with it not being merely adventitiously associated with protein. These data provide experimental support for the presence of nucleation centers for the mineral cores in bacterioferritins and indicate that these proteins are not simply acting as vessels in which hydrolysis of Fe(3+) occurs independent from the protein surface. From analyses of X-ray structures and amino acid sequence comparisons, possible nucleation sites are identified.