Iron, insoluble as free Fe(3+) and toxic as free Fe(2+), is distributed through the body as Fe(3+) bound to transferrin (Tf) for delivery to cells by endocytosis of its complex with transferrin receptor (TfR). Although much is understood of the transferrin endocytotic cycle, little has been uncovered of the molecular details underlying the formation of the receptor-transferrin complex. Using cryo-electron microscopy, we have produced a density map of the TfR-Tf complex at subnanometer resolution. An atomic model, obtained by fitting crystal structures of diferric Tf and the receptor ectodomain into the map, shows that the Tf N-lobe is sandwiched between the membrane and the TfR ectodomain and that the C-lobe abuts the receptor helical domain. When Tf binds receptor, its N-lobe moves by about 9 A with respect to its C-lobe. The structure of TfR-Tf complex helps account for known differences in the iron-release properties of free and receptor bound Tf.