With the purpose to elucidate the in vivo aging processes of erythrocytes as a follow-up study of the previous report, the author studied the changes of easily split off iron, resistance to osmotic pressure and mechanical fragility in the labeled erythrocytes, young, mature, and aged, obtained after intravenous injection of Fe(59)-globulin into blood-depleted anemic rabbits and further given a large dose of non-radioactive iron compound; and obtained the following results. 1. In the observations conducted on changes in the easily split off iron of labeled erythrocytes from young ones to aged ones after intravenous injection of Fe(59)-globulin into blood-depleted anemic rabbits, the easily split off iron content is greatest in the regenerating stage and least in the mature stage; whereas in the aging stage it again tends to increasa slightly. Furthermore, these fluctuations are considered to ba stochastically significant changas. 2. In comparing the percentage of residual erythrocytes with the change in the easily split off iron content in peripheral blood after the transfusion of labeled erythrocytes, young, mature, and aged, into normal rabbits, generally there can be recognized an inverse relationship between the two, and it has been reconfirmed that the content of easily split off iron in erythrocytes, including some young erythrocytes destined to an early destruction increases along with the aging. 3. In estimation of osmoticpressure resistance of erythrocytes in various age by the hemolytic ratio in hypotonic saline solution, it has been found that the resistance to osmotic pressure is weakest in young erythrocytes, and it increases rapidly with maturation, reaching equal to or slightly over the resistance of whole erythrocytes. Moreover, no significant changes in the osmoticpressure resistance can be observed along with aging processes, and the resistance of aged erythrocytes is about the same as that of whole erythrocytes. 4. In estimating mechanical fragility of erythrocytes in similar age range, by the ratio of hemolysis due to shaking, the fragility is greatest in aged erythrocytes, followed by young ones and least in mature ones.