Abstract: A method for the measurement of serum ferritin by radioimmunoassay is present-ed. ¹²⁵I-labelled human liver ferritin and rabbit antiferritin antibody were used. Goat anti-rabbit gamma globulin antibody. together with polyethylene glycol, was used as the separating reagent. The working range was up to 320 ng/ml. The method was simple to handle, precise, and sensitive. and it was therefore thought to be suitable for routine diagnostic measurement of ferritin. Serum ferritin values for a population of men, wom-en. and women older than 50 years were 104.3±5.8. 30.5±1.7, and 64±6.8 ng/ml respectively （mean±SE）. The blood donors from rural areas had much lower values. Eight of ten patients with iron deficiency anemia had significantly lower values （＜5. 4ng/ml）. Ferritin concentrations were greatly increased in three patients with acute myelocytic and promyelocytic leukaemia.