1. Erythroblasts were enriched from human bone marrow samples and fractionated on Percoll gradients according to maturity.
2. Heart-type and spleen-type ferritin was measured in each fraction by an immunoradiometric assay.
3. In normal marrow, heart-type ferritin content was higher in the early erythroblast fractions and fell with maturation. Spleen-type ferritin content showed no such consistent change.
4. Megaloblastic erythroblasts had a significantly higher ferritin content.