The role of the transferrin receptor and of cell proliferation during human renal embryogenesis has been studied by using immunocytochemical techniques. Tissue sections of human fetal kidneys have been stained using monoclonal antibodies to the cell surface transferrin receptor (HB21) and the nuclear proliferation antigen (Ki67). Both antigens appear during nephron induction, are strongly expressed during the early stages of renal tubulogenesis, but are lost with progressive maturation. Simultaneous changes in the intermediate filament phenotype at this developmental stage have also been demonstrated by the use of monoclonal antibodies to vimentin and low molecular weight cytokeratin. This study provides immunocytochemical evidence that in the human kidney, as in in vitro development of the mouse kidney, one of the early events in tubule induction involves transferrin-dependent proliferation.