For the purpose of clarifying the significance of E S Fe, starch block electrophoresis of rabbit hemoglobin was been carried out and the following results were obtained. 1. The hemoglobin was separated in three fractions, Ⅰ, Ⅱ and Ⅲ. In the migration distances, these fractions corresponded to Hemoglobin A(1), A(2) and A(3) of adult hemoglobin respectively. 2. E S Fe was detected most in the fraction Ⅲ (largest at mobility), while free iron in that fraction was less than the other fractions. 3. In estimating the ratio of Fe(59)-radioactivity to hemoglobin volume (mg) of the Fe(59) labeled hemoglobin solution in the each fraction eluate, it was noticed that the specific activity of the fraction Ⅲ was less than those of others during 3 weeks after the administration of Fe(59), followed by a significant increase after 6 weeks. It was thought from the above that the hemoglobin in fraction Ⅲ different from that in the other fractions, and that in the early stage was different from that in the later one. 4. The ratio of radioactivities in easily split off iron to that in hemoglobin iron of the same samples as the above increased markedly in the fraction Ⅲ in the early stage, followed by a gradual decrease during 2 weeks and again a tendency of increase at 4 weeks later in the same fraction. 5. It was indicated from these results that the characters of the globin in new formed hemoglobin and aged one were different from that in mature one, and that this difference in the characters was resulted in an increase of E S Fe due to hemoglobin degradation.