A direct radioimmunoassay for ferritin in serum is described in which Bolton and Hunter reagent is used to label ferritin. The detection limit of the assay is 150 pg; 95% reference ranges were found to be 12-200 microgram/l for men and 5-76 microgram/l for women. Ferritin concentrations in patients with iron deficiency anaemia were found to be uniformly low in subjects with uncomplicated iron deficiency but were normal or even raised in subjects with iron deficiency anaemia associated with malignant or inflammatory conditions.