1. The rate of in vivo release of iron from plasma transferrin samples at low and high saturation was measured simultaneously in fifteen subjects with differing amounts of body iron and different rates of erythropoiesis. Iron atoms bound to transferrin at low and high plasma iron saturation were identified by using tracer labels 55Fe and 59Fe respectively. Results were expressed as the plasma 55Fe and 59Fe half-clearance time. 2. Plasma iron clearance times ranged from 15·6 to 350 min, reflecting the different amounts of available iron and marrow requirements of the patients studied. The simultaneous rates of 55Fe and 59Fe clearance from the plasma were the same in thirteen patients. Clearances in two subjects showed differences between the two isotopes, but clearances of the identical paired isotopes in two other subjects showed no difference. 3. These results confirm earlier findings of homogeneity in the plasma iron pool, extending the observations to include varying per cent saturation of transferrin and varying rates of iron uptake.