IT was not until recently that a method for the quantitative estimation of erythropoiesis became available1. The method introduced requires the application of high doses of radioactive iron, a very stable and sensitive measuring equipment and protracted measurements. The authors showed that the usual calculation of erythrocyte formation from plasma iron-turnover (PIT) values is frustrated by a substantial return of radioiron from the bone marrow into the plasma, starting at about 8 h after injection of radioiron. This finding and the results of Lamerton et al.2 induced me to block the re-utilization of radioiron by injecting inactive iron. Under such conditions PIT would correctly measure iron uptake by the organs of the body, and the radioiron incorporation into erythrocytes would be a correct indicator of the amount of iron built in into erythrocytes. The quotient between the amount of iron contained in the total circulating haemoglobin and the amount built in into erythrocytes would then be a measure of the effective life-time of the erythrocytes.