The in vitro selfassembly of apoferritin after previous dissociation and unfolding in 7.2M guanidinium chloride, pH 3.5, yields up to 80% of a protein complex exhibiting the molecular mass of the native icositetramer of greater than or equal to 450 kDa. After removal of high molecular mass byproducts, the final reassembly product proves to be indistinguishable from native apoferritin with respect to its functional and conformational properties. These refer to the intrinsic fluorescence and to the far and near UV circular dichroism. The unfolding transitions of the native and reassembled protein in aqueous guanidinium chloride or at acid pH coincide within the range of error. The reassembled protein is also able to catalyze the oxidation of Fe(II). Higher polymers of the apoferritin complex represent most of the residual 20% of the reconstituted protein. They are stabilized by non-covalent (preferentially hydrophobic) interactions, and may be disassembled to the icositetramer by preferential solvation of the protein in the presence of less than or equal to 50% (v/v) ethylene glycol. The change in fluorescence emission accompanying polymerization reflects altered surface properties of the apoferritin subunits compatible with those reported for the ferritin----hemosiderin transition.