Purified human spleen ferritin was labelled with 125I. On Sepharose 6-B gel filtration four species of labelled products were separated: a component with a higher molecular weight than ferritin; a component which is eluted in the same volume as unlabelled ferritin; and two labelled compounds with molecular weights lower than ferritin. When these labelled materials were used in a double antibody radioimmunoassay, the high molecular weight fraction showed variable and high non-specific binding and was poorly displaced by unlabelled ferritin; the fraction behaving like true ferritin gave good standard curves and showed non-specific binding of less than 1%. The remaining two components showed poor binding to rabbit antiferritin. Using labelled material from the second fraction, a double antibody radioimmunoassay capable of measuring 2 microgram ferritin protein/litre of serum was developed. Inter- and intra-assay variation was between 3% and 8% over a concentration range of 0 to 250 microgram ferritin protein/litre. Good agreement between serum ferritin levels assayed by the present method and by an immunoradiometric method was obtained. Labelled ferritin was stable for at least six weeks. The simplicity of the methodology makes it possible to assay serum ferritin in large batches.