Bovine embryonic trachea cells were hybridized with mouse A9 cells deficient in hypoxanthine phosphoribosyltransferase, and cattle-mouse hybrid cell clones were isolated after HAT/ouabain selection. In these interspecific cell hybrids, bovine glucose-6-phosphate dehydrogenase, α-galactosidase, and phosphoglycerate kinase were expressed concordantly with bovine HPRT. Their expression depended on the presence of the bovine X chromosome. These data indicate that the genes for G6PD, PGK, and HPRT are linked and can be assigned to the bovine X chromosome