Serum ferritin concentration was measured by immunoradiometric assay in 64 subjects. It was closely related to the size of body iron stores measured by hemosiderin content of bone marrow in all subjects and by the deferoxamine test in 10 patients with iron overload. Urinary cobalt excretion, an indirect measure of iron absorption, was inversely related to hemosiderin content of bone marrow in 34 patients aged 18 to 72 with or without liver disease, but this relation did not hold in a group of 20 student volunteers aged 17 to 30, indicating that the test is unreliable in young people. A strong inverse correlation was demonstrated between values for cobalt excretion and serum ferritin in the 34 patients and between those for iron absorption and serum ferritin in the 20 students. Serum ferritin concentration appears to reflect accurately the iron status of the healthy individual but high values in liver disease must be interpreted with caution.