We describe the development and evaluation of a serum ferritin radioimmunoassay, in which 125l-labeled ferritin and rabbit anti-ferritin antibody are used. Goat anti-rabbit gamma-globulin antibody, together with polyethylene glycol, is used as the separating reagent. The assay has a working range up to 500 mug of ferritin per litre, and a sample requirement of 75 mul of serum for assay at two dilutions. The assay requires 24 h. it has a sensitivity of 1.5 mug of ferritin per litre and a long-term precision (CV) of 13%. Reference intervals for a population of men were 18-330 mug/litre, with no marked age dependence, while those for a population of women older than 50 years were 18-200 mug/litre. Many apparently healthy women in the 20-50 year age group have much lower concentrations. Serum ferritin concentrations of less than 18 mug/litre are indicative of iron deficiency, defined as the absence of stainable iron in an aspirate of bone marrow.