The molecular basis of the transferrin (TF)–transferrin receptor (TFR) interaction is not known. The C-lobe of TF is required to facilitate binding to the TFR and both the N- and C-lobes are necessary for maximal bind-ing. Several mAb have been raised against human transferrin (hTF). One of these, designated F11, is specific to the C-lobe of hTF and does not recognize mouse or pig TF.