Lactotransferrin (LTF) is an important member of the transferrin family of proteins. These proteins play an essential role in the transport of iron in extracellular fluid (Aisen and Lis-towsky, 1980). Southern blot analysis of mouse-human somatic cell hybrids have localized the LTF gene to region q21→qter of human chromosome 3 (Teng et al., unpublished data). Using the same full-length mouse cDNA probe (2.2 kb), the LTF gene was mapped to human chromosomal bands 3q21→q23 by in situ hybridization. The sublocalization of the LTF gene to 3q21→q23 is in the region of human chromosome 3 where the gene loci of transferrin and transferrin receptor have been localized (Yang et al., 1984; van de Rijn et al.,