Intracellular transport and storage of iron in bone marrow reticulum cells can be studied in the electron microscope by the recognition of iron macromolecules. Following artificially increased bone marrow erythrophagocytosis ferritin molecules accumulate both in the cell sap and in the lysosomes of erythroclastic reticulum cells. Persistent iron loading transforms lysosomes into siderosomes, after which haemosiderin may be formed by gradual transformation of ferritin. The bone marrow reticulum cell shows evidence suggestive of autophagy, a process which may allow for the transfer of cell sap ferritin into lysosomes, as documented for other types of iron storing cells.