Summary.Monocytes, lymphocytes and polymorphs were separated from the peripheral blood of normal human subjects. Ferritin concentrations were determined with antibodies to both human spleen and heart ferritins. The heart type ferritin concentration in monocytes was 38·4±21·6 fg/cell (mean ±SD), in lymphocytes 8·6±6·6 fg/cell and in polymorphs 3·2±2·4 fg/cell. Spleen type ferritin concentrations (fg/cell) were 15·6±7·0 in monocytes, 6·6±5·7 in lymphocytes and 7·0±4·6 in polymorphs. The mean heart/spleen ferritin ratios were 2·8/1 for monocytes, 2·0/1 for lymphocytes and 0·6/1 for polymorphs. The cell extracts were also subjected to anion exchange chromatography. Heart type ferritin eluted at a higher chloride ion concentration than spleen type ferritin. Both H and L subunits were synthesized by mononuclear cells when incubated with3H‐leucine. Human leucocytes contain a wide range of isoferritins and ferritin concentrations may be considerably underestimated in conventional assays for serum ferritin which employ antibodies to l
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