A cDNA clone, RhIXb (1384 bp), encoding the entire protein sequence of a human blood group Rh polypeptide has been used to map the Rh locus, by in situ hybridization, to the region p34.3-p36.1 of chromosome 1. Two other unrelated cDNA clones, pUCA2 (750 bp) and pUCIII (1600 bp), isolated during the cloning procedure of the Rh cDNA were investigated simultaneously, and assigned to chromosome 3p21.1-3p22 (clone pUCA2) and to chromosome 22q12.1-22q13.1 (clone pUCIII).