It may well be that the main value of these studies will be to focus interest on the remarkable properties of the red cell and to stimulate yet more work on its ultramicroscopic structure and biochemical properties. As more immediate gains, they have given us improved methods of blood storage, better diagnosis of the haemolytic anaemias and more understanding of the mechanism of diseases such as polycythaemia vera. For the clinical scientist they have opened up new lines of work within the competence of those of us whose task it is to study the organism as a whole. The volume of accurate metabolic work on diseases of the blood is small. This is not from lack of requirement, for we know little about the mechanism and treatment of the common forms of anaemia from trauma, infection and malnutrition. It has rather been because of the formidable nature of the task, the difficulties imposed by the facts that much of the metabolism, as for example that of iron, goes on in a closed circle and that the level of the blood is affected by the two variables of production and destruction. Now that we can measure the rates of blood formation and blood destruction independently, with reasonable accuracy and simplicity, I think we may well see a recrudescence of interest in the metabolic problems of anaemia and blood regeneration, and one may hope that it will be as fruitful as the application of quantitative methods has proved in other fields of metabolism.