Ferritins are a group of isometric proteins having an important function in iron storage and metabolism and are found in high concentration in the liver, spleen and bone marrow. Acidic isoferritins are found in human fetal liver, primary mammary, gastric and pancreatic carcinomas, and are termed carcinofetal ferritins. Elevated levels of serum ferritin were found in patients with various malignant diseases such as Hodgkin's disease, chronic myeloblastic, granulocytic and lymphatic leukemias and myeloblastosis, in patients with breast cancer, multiple myeloma, malignant lymphoma, carcinoma of the gastro intestinal tract and germinal cell tumors of the testis. Recently a subpopulation of circulating T lymphocytes bearing surface ferritin was found in patients with breast cancer and untreated Hodgkin's disease. No such lymphocytes were demonstrated in normals or in patients with benign breast disease. The appearance of such subpopulation in the circulation is an early manifestation of the neoplastic disease, and its identification may provide a tool of potential diagnostic and prognostic importance in the management of Hodgkin's disease and breast cancer.