The uptake and release of 131I-labelled diferric transferrin by rat reticulocytes was examined both in vitro and in vivo. Cycle time in vitro was estimated to be 2.5 min in iron-deficient reticulocytes and 2.3 min in phenylhydrazine-produced reticulocytes. In vivo reticulocyte uptake and release of labelled diferric transferrin injected in the iron-deficient rat averaged 1.7 min.
