It has been found possible to test ferritin concentrations in the reticulum and hemopoietic cells from human bone marrow by an immunoradiometric assay. Ferritin concentration in healthy test persons amounts to 0.92 ± 0.38 ng/μg protein in the reticulum and 0.084 ± 0.031 ng/μg protein in hemopoietic cells. In healthy test persons about 90% of the ferritin is localized in the reticulum and about 10% in the hemopoietic cells. The ferritin concentration in the reticulum is decreased in patients having iron deficiency anemia. In these patients only 50–70% of the bone marrow ferritin is localized in the reticulum. Bone marrow ferritin was characterized by a combination of anion-exchange chromatography using Sephadex A-50 and an immunoradiometric assay. Marrow ferritin from healthy persons is eluted at chloride concentrations between 200 and 300 mM. The anion-exchange chromatographic properties of ferritin in the reticulum and hemopoietic cells are iden