The kinetics of the intracellular iron were studied in an invitro system using reticulocyte-rich rabbit red cells incubated with Fe59-labeled plasma. The incubation was stopped at intervals of a few seconds to 70 minutes, and the cells were fractionated by differential centrifugation and by chemical means. In being selectively taken up by reticulocytes, Fe59 is first associated with particulate fractions and then gradually released to the soluble cytoplasm, when the iron is incorporated into hemoglobin and into a transient nonhemoglobin protein phase. The entire process from the initial binding of iron by cell surfaces receptors until the incorporation of iron into hemoglobin requires only 6 to 8 minutes at 37 C. It is probable that in suitable concentrations, lead allows Fe59 to accumulate in stroma but blocks its entry into the nonhemoglobin protein iron phase and into hemoglobin.

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