1. The properties of ferritin in serum have been compared with those of ferritin from a number of tissues including blood cells. On anion-exchange chromatography with DEAE-Sephadex, the behaviour of human heart ferritin is different from that of liver, kidney or spleen ferritin. Reticulocyte ferritin appears to have similar characteristics to heart ferritin. 2. Serum ferritin from normal subjects and patients with various degrees of iron load, leukaemia or liver disease all have a much lower affinity for the anion-exchange column that any tissue ferritin, suggesting a difference in isoelectric point. The elution point of serum ferritin from patients with acute myeloblastic leukaemia is significantly different from normal. 3. Density gradient centrifugation in sucrose showed that ferritin in leucocyte extracts and partially purified ferritin from the serum of two patients with iron overload behaved as apoferritin rather than the iron-rich protein. 4. The results suggest that ferritin is modified during its entry into the plasma and that even in cases of iron overload the iron content of serum ferritin may be low. The findings are of importance in considering the origin of plasma ferritin, the clearance of ferritin from plasma and its role in iron metabolism.