The complete gene encoding the human lactoferrin (hLf) was isolated from a cosmid library and its structure was characterized. A detailed restriction map and the complete exon-intron structure of the gene are presented. The transcript is assembled from 17 exons like the bovine (bLf) and murine lactoferrin (mLf) genes, which are separated by introns ranging in size from about 300 bp to 3.3 kb. The total length of the gene is about 24.5 kb, estimated by adding up the sizes of the exons and introns. A comparison of the structure of the hLf gene with that of other species revealed that the hLf gene is more closely related to bLf than to mLf. The positions of the introns were well matched with those of the bLf gene except only for exons 14 and 15, wherein exon 14 of the hLf gene is one nt shorter while exon 15 is one nt longer than the bLf gene. All the exon-intron boundaries of the gene conformed to the GT/AG rule. In situ fluorescence hybridization indicated that the hLf gene was mapped in the region 3p21.3.