By creating various changes mainly in erythroblasts of adult rabbits and by administering iron to these animals, changes in the proportion of sideroblasts have been studied comparatively; and the iron intake by erythroblasts has been examined. The following are the results of the study: 1) There seems to be a limitation in the iron intake by erythroblasts, namely, the iron intake is not solely dependent on the quantity of iron supplied by serum, but also it is influenced greatly by the quantity of nonhemoglobin iron controlled by the amounts of hemoglobin in the erythroblasts matured to a certain stage. 2) It is believed that the mechanism of iron intake is of a relatively simple nature something like diffusion, i.e., the degrees of concentration of the amounts of iron supplied by serum on one side and nonhemoglobin iron in erythroblasts on the other side, of the cellwall. 3) In erythroblasts, three steps of the metabolic process, namely, intake, retention and utilization of iron, essentially take place almost simultaneously, mainly in the later maturation phase.