Southern blot analysis of genomic DNA and gene-cloning experiments have recently shown that the human ZP3 gene is not a single-copy gene. The human genome harbors sequences encoding a protein of 424 amino acids and, in addition, a polymorphic locus with the potential to give rise to a probably nonfunctional polypeptide of 372 residues. In this report it is shown, by screening of a panel of human x hamster hybrid cell lines, that both the ZP3 and ZP3P loci are located on human chromosome 7.