Analysis of radioactive iron kinetics was performed using a multi-compartment model on a series of 300 patients having either a quantitative (aplasia, haemolysis, iron deficiency) or qualitative (dyserythropoiesis, agnogenic myeloid metaplasia) anomaly of iron metabolism. Calculations were performed using a mammillary model of iron metabolism. The study demonstrated that the flux of iron from the plasma to the exchangeable compartment was a constant fraction of the global iron flux, equal to 15%, in those cases without dyserythropoiesis. This suggested that a constant correction for calculations of iron flux from the slope of the initial portion of the radioactive iron elimination curve may be applied to calculations of haemoglobin production and to the movements of iron to its exchangeable pools. Contrary to previously published information, the exchange of iron between the plasma and the exchangeable pools was not related to circulating iron levels when the other parameters were held constant. In the patients with aplastic anaemia the iron flux was diminished, but never eliminated, demonstrating that the exchangeable compartment was not solely erythroblastic, but included non-erythroid transferrin receptors. In dyserythropoietic states and myelofibrosis, the iron flux from the plasma was elevated, indicating that an important fraction of radio-iron leaves the plasma and then returns, without participating in effective erythropoiesis as determined by the appearance in the circulation of labelled viable red blood cells. The determination of this movement permitted the measurement of ineffective erythropoiesis.