Some malignant tissues and cell lines contain acidic isoferritins and it has been suggested that the assay of such isoferritins in serum may be of value in the diagnosis of malignancy. This paper describes a radioimmunoassay for acidic ferrtin purified from HeLa cells. Examiniation of purified heart, kidney, liver and spleen ferritin showed that the assay was highly specific for acidic isoferritins. Ferritin concentrations have been measured with antibodies to HeLa cell and spleen ferritin in extracts of normal and tumour tissue. Although the tumours contained more HeLa type ferritin than the corresponding normal tissue the HeLa/spleen type ferritin ratio was low. HeLa-type ferritin concentrations have been compared with values obtained with anti-spleen ferritin in over 1000 sera from normal subjects and patients with cancer and leukaemia. HeLa-type ferritin as not detected ( less than 2 micrograms/l) in most normal sera. Concentrations of up to 53 micrograms/l were found in sera from patients with malignant disease but the HeLa/spleen type ferritin ratio was always very low. There appears to be little application for antibodies to HeLa cell or heart ferritin in the diagnosis or monitoring of cancer.