It seems likely that iron which has crossed the cell membrane and has been released from transferrin enters a labile intermediate pool from which it is available for haem synthesis, for the activation of iron-dependent enzymes, for incorporation into ferritin or for a return to extracellular transferrin. Enlargement of this pool stimulates ferritin synthesis. Iron probably enters the transit pool not only from transferrin but also as a result of endogenous haem breakdown and the mobilization of ferritin iron. Evidence for the occurrence of the transit pool has been obtained for reticuloendothelial cells, red cells precursors, cultured Chang cells and liver. It is suggested that the transit pool consists of a low molecular weight complex and that this is a major source of the iron chelated by agents such as desferrioxamine. No more precise characterization has been possible up to the present time.