Human ferritin homopolymers with H or L subunits (rHF and rLF) were genetically engineered in E coli. Apoferritins were then reconstituted with 2000 Fe atoms. A big difference was observed in the rates of iron uptake, whereas the mean core size was similar in rHF and rLE Magnetization of the recombinant human ferritins were measured as functions of temperature and field. The blocking temperature T-B(H) at low fields is considerably higher in rLF than in rHF. From the fit of M(H) data to a modified Langevin function: M(H)=M0L(mu(p)H/k(B)T)+XaH, the effective magnetic moment mu(p) is found to be much larger in rLF than in rHF. Experimental data demonstrate that the magnetic properties, in particular, the uncompensated spins of ferritin core are related to the biomineralization process in ferritins. (c) 2005 American Institute of Physics.