1. Changes in plasma and tissue ferritin concentrations were surveyed in mice fed on iron deficient diets (Fe, 2.4 micrograms/g) from the age of 8 to 30 weeks by an enzyme-immunoassay for mouse ferritin. Values were compared with those in mice on control diets (Fe, 110 micrograms/g) and mice with blood loss. 2. In mice on iron deficient diets, while plasma ferritin remained at normal level, ferritin in the spleen and kidney decreased, but that in the liver increased as much as twice of that in the control. 3. Mice with blood loss revealed a drastic reduction of ferritin in every tissue and plasma examined.