Human tissues contain ferritin molecules with a range of isoelectric points but immunoassays for detecting serum ferritin have generally employed antibodies to the more basic liver or spleen proteins. To study the distribution of more acidic ferritins in tissues and serum acidic ferritin has been isolated from normal human heart and a two-site immunoradiometric assay for this protein developed. This assay gives little cross-reaction with spleen ferritin. Tissue ferritins have been fractionated by anion exchange chromatography and assayed with both spleen and heart antibodies. The spleen ferritin assay detects the more basic ferritin and the heart ferritin assay the more acidic ferritin. Acidic ferritins were found in heart, kidney, reticulocytes and HeLa cells. In sera from normal subjects and patients with iron overload, myocardial infarction, leukaemia and carcinoma only low concentrations of heart ferritin were found, although in the pathological sera spleen ferritin concentrations were generally raised. Circulating ferritin contains only a small proportion of molecules with the immunological characteristics of acidic heart ferritin.