To understand the mechanism underlying the preferential dimerization of ferritin shells, we studied monomers and dimers from both horse spleen and recombinant horse L-apoferritin by using gel filtration, nuclear magnetic resonance, electrophoresis, transmission electron microscopy, and gene engineering techniques. Our study of the kinetics of dimer-monomer dissociation that is produced by heating revealed the presence of at least two types of dimers, namely, weakly and strongly linked dimers with activation energies of 124 +/- 14 and 157 +/- 16 kJ/mol, respectively. Our study using thiol reagents indicated that the dimerization in horse spleen ferritin is partially mediated by disulfide bridges being formed between H-chains. Our analysis of the components that resulted from the dimer-monomer dissociation further clarified that these dimers form interdigitation structures. In summary, five types of dimers were identified in horse spleen apoferritin: reversible dimers with very weak interaction, non-sulfide dimers with weak interaction, non-sulfide dimers with strong interaction, disulfide dimers linked only by disulfide bridges, and disulfide dimers linked by disulfide bridges and having other interactions.