Transport of iron across the cell membrane is mediated by the iron-binding serum protein, transferrin, and its cell-surface receptor. Transferrin receptor is required for cell proliferation and may play a functional role in the pathogenesis of iron-storage disorders and some neoplasias. To better understand the possible involvement of transferrin receptor in such disorders, we have determined the chromosomal locus of the receptor gene by in situ hybridization. The human transferrin receptor gene was thus mapped to 3q26.2----qter, a region of chromosome 3 that appears to be involved in metal transport and that is subject to nonrandom structural rearrangements associated with neoplasia.