Electron microscopy has shown that normal mouse liver cells contain abundant cytosolic and lysosomal holoferritin but none in cisternae of the rough endoplasmic reticulum (RER). This does not mean that ferritin is absent from the secretory pathway, since the low contrast of apoferritin and ferritin containing little iron makes it difficult to resolve using standard transmission electron microscopy. We hypothesized that treatment of permeabilized cells and cell fractions with iron should make apoferritin visible by converting it to holoferritin. The iron treatment caused particles resembling the cores of holoferritin to appear in RER and in RER microsomes from mouse liver. We conclude that ferritin occurs in the endoplasmic reticulum and could be secreted, allowing the liver to be a source of serum ferritin.