Dividing cells require iron and, therefore, express the transferrin receptor (CD71) on the cell surface to enable internalization of transferrin-bound iron. Since early T cell development is marked by intense proliferation, we questioned whether CD71 might serve as a marker of immature T cells. Therefore, we analyzed the expression of CD71 on fetal, neonatal, and adult thymocytes in correlation with cell size, cell cycle status, and expression of CD3, CD4, CD8, alpha beta TcR, and gamma delta TcR. Phenotypic analysis showed that only the large, immature CD4-8-3-, CD4-8+3-, and CD4+8+3- cells in fetal, neonatal, and adult thymus expressed CD71. In addition, DNA analysis showed that all CD71+ large adult thymocytes were cycling. Downregulation of CD71 occurs when proliferation ceases, i.e., within the CD4+8+3- thymocyte subpopulation. The gradual changes in size and CD71 expression suggest a sequential development within this CD4+8+3- subpopulation from large CD71+ via small CD71+/- to small CD71- cells. As a consequence, CD71 expression is downregulated, in adult T cell development as well as in ontogeny, before the alpha beta TcR appears on the cell surface of the thymocyte. Together, our findings show that CD71 is a marker of immature, proliferating T cells.