A series of marsupial-eutherian somatic cell hybrids was produced by fusion between lymphocytes from the red kangaroo (Macropus rufus) and HPRT-deficient mouse cells. The hybrids lost marsupial chromosomes and could therefore be used to map marsupial genes. Several of the hybrids contained a complete red kangaroo X chromosome and expressed the kangaroo form of the enzymes HPRT, G6PD, and PGKA. A number of HPRT-deficient revertant cell lines were derived from the hybrids. These possessed a variety of partially deleted X chromosomes. With these cell lines, it has been possible to establish the X-linkage of the genes for HPRT, G6PD, and PGKA in this marsupial and to localize these three genes to the terminal portion of the euchromatic arm of the red kangaroo X chromosome.