Ferritin is an iron storage protein of high-molecular weight which is primarily present in the liver, spleen, and bone marrow. A very sensitive immunoradiometric assay has been developed which permits determination of serum concentrations in normal persons and in patients with a variety of different disorders. In normal subjects, the serum ferritin concentration correlates very well with total body iron stores as measured by phlebotomy. The serum ferritin concentration is reduced in patients with iron-deficient anemia and is significantly higher in patients who are anemic for other reasons. Subject areas discussed in this review include the details of the immunoradiometric procedure, the sensitivity and accuracy of the assay, factors influencing the assay, values characteristic of a variety of clinical disorders, and the utility of the assay in clinical medicine and public health.