1case the peak is reached a t the polychromatic stage (94%), and higher activity is revealed in the basophilic erythroblasts. These data should be considered in relation to relatively earlier sampling (3 hours), the higher radioiron dosage (100 pc), and the stimulation of bone marrow by previous bleeding.The present investigation then shows that in the rat and with the doses used, there is a definite turnover of radioiron even by the earliest erythroid elements, namely proerythroblasts and basophilic erythroblasts. This was not known before, and it can be assumed as indirect evidence of the earliest stages of hemoglobin formation in the marrow.Summary. The presence of iron-59 in the earliest stages of the erythroid series of the rat bone marrow was demonstrated by means of stripping film autoradiography after intraperitoneal injection of labeled ferric chlorides. Pro-erythroblasts and basophilic erythroblasts take up a detectable amount of radioiron after 3 to 6 hours. The maximum uptake, however, occurs in the polychromatic and orthochromatic stages.

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