The possible use of desferrioxamine (DF) to measure body iron stores necessitated studies on the source of the iron excreted in the urine with the compound. DF did not take significant amounts of iron from transferrin because:(1) DF did not affect the plasma iron turnover(2) The excretion of a transferrin‐bound radioiron tracer was not increased by DF(3) Induced elevation of the plasma iron level was followed only by a slight increase in the iron excretion(4) No effect was observed on the incorporation into red cells of a transferrin‐bound radioiron tracer by DFOther studies indicated that iron, in the stage of being incorporated into red cell precursors, was not a probable source of the iron excreted with DFA significant delay in the excretion of iron with DF was regularly observed and found to be due to a delayed mobilisation of ironFurther studies are necessary to find the source of iron excreted after administration of