The PGK activity was assayed in diploid, hyperdiploid, tetraploid, and hybrid cells all originating from the same Chinese hamster cell line (DON line). A relationship between gene multiplicity and enzyme activity was observed. Selective pressure on the HGPRT locus by growth of hybrid cells in the presence of 8-azaguanine resulted in decreased levels of PGK activity. Growth of these hybrids in the presence of 5-BUdR did not influence the enzyme activity. It was concluded that the genes coding for HGPRT, PGK, and G6PD are linked in the Chinese hamster. The TK locus seems to be linked neither to the HGPRT, PGK, and G6PD loci nor to the 6PGD locus.