Existing ferrokinetic methods do not provide a direct and quantitative measurement of effective and ineffective red cell production. A new method is described for calculating the daily uptake of iron by maturing red cells and the mean red cell lifespan. Ineffective erythropoiesis and non-erythroid iron turnover are also measured. The method involves standard laboratory techniques but the analysis requires access to a computer. The preliminary results suggest that it will be a clinically useful tool for the investigation of erythroid disorders.