Herein is reported an anti-transferrin receptor antibody that specifically binds to human transferrin receptor and cynomolgus transferrin receptor, which comprises i) a humanized heavy chain variable domain derived from the heavy chain variable domain of SEQ ID NO: 01, and ii) a humanized light chain variable domain derived from the light chain variable domain of SEQ ID NO: 26, wherein the antibody has an off-rate for the human transferrin receptor that is equal to or less than (i.e. at most) the off-rate of the anti-transferrin receptor antibody 128.1 for the cynomolgus transferrin receptor, whereby the off-rates are determined by surface plasmon resonance, and whereby the anti-transferrin receptor antibody 128.1 has a heavy chain variable domain of SEQ ID NO: 64 and a light chain variable domain of SEQ ID NO: 65.
