Iron uptake and micelle formation in ferritin and apoferritin have been followed both spectrophotometrically and by means of sedimentation velocity experiments. Information was thus obtained on the molecular weight distribution of the reconstitution product. To achieve incorporation 'native' ferritin (whole ferritin as purified from horse spleen), 'native' apoferritin (apoferritin prepared by fractionation of ferritin preparations) and 'reduced' apoferritin (apoferritin prepared by reduction of ferritin by dithionite or ascorbic acid) have been incubated with ferrous salts in the presence of oxidizing agents under different experimental conditions. Although some iron is incorporated in 'native' ferritin, full saturation is not achieved and the molecular weight distribution of the incubated products remains heterogeneous. 'Native' and 'reduced' apoferritin show a similar iron incorporation, but the reconstitution products markedly differ in terms of their iron distribution. Ferritin reconstituted from 'native' apoferritin has a broad molecular weight distribution, while that reconstituted from 'reduced' apoferritin is characterized by a narrow, homogeneous molecular weight distribution. However treatment of apoferrition with reducing or oxidizing agents prior to the incubation alters the characteristics of the iron distribution without changing the iron incorporation properties. These results point to a role of the protein moiety not only in iron oxidation, but also in micelle formation.