We used chromatographic, static and dynamic light scattering techniques, and atomic force microscopy (AFM) to study the structure of the protein species and the protein-protein interactions in solutions containing two apoferritin molecular forms, monomers and dimers, in the presence of NaAc buffer and CdSO4. The sizes and shapes of the monomers and dimers, separated by size-exclusion chromatography, were determined by dynamic light scattering and AFM. While the monomer is an apparent sphere with a diameter corresponding to previous X-ray crystallography determinations, the dimer shape corresponds to two, bound monomer spheres. Static light scattering was used to characterize the interactions between solute molecules of monomers and dimers in terms of the second osmotic virial coefficients. The addition of even small amounts of Cd2+ causes attraction between the monomer molecules. Furthermore, we found that the second virial coefficient and the protein solubility do not noticeably depend on temperature in the range from 0 degreesC to 40 degreesC. This suggests that the enthalpy for crystallization of apoferritin is close to zero, and the gain of entropy is essentially constant in this temperature range. We also found that in solutions of the apoferritin dimer, the molecules attract even in the presence of acetate buffer only, indicating a change in the surface of the apoferritin molecule. In view of the repulsion between the monomers at the same conditions, this suggests that the dimers and higher oligomers form only after partial unfolding of some of the apoferritin subunits. These observations suggest that aggregation and self-assembly of protein molecules or molecular subunits may be driven by forces other than those responsible for crystallization in the protein solution.