In man-Chinese hamster somatic cell hybrids the segregation patterns of the loci for 25 human enzyme markers and human chromosomes were studied. The results provide evidence for the localization of the gene for adenylate kinase-1 (AK1) on chromosome 9. Since the loci for the ABO blood group (ABO), nail-patella syndrome (Np), and AK1 are known to be linked in man, the ABO:Np:AK1 linkage group may be assigned to chromosome 9.