The Chinese hamster genes ADK, NP, ESD, PGM2, PEPS, PEPB, GLO, and GSR, all of which are on Chinese hamster chromosome 1, were assigned to CHO-LA chromosomes by analysis of the segregation of CHO isozymes and chromosomes from interspecific somatic cell hybrids made with CHO cells and mouse C11D cells. One allele of each of these eight loci remained linked on the normal chromosome 1 homolog. For seven loci, the other allele remained linked on chromosome Zl, but ESD was shown to have been translocated to chromosome Z6 (Chinese hamster chromosome 5q+). Ag-NOR staining of CHO chromosomes indicated that the (1; 5) translocation was very likely reciprocal, since the Chinese hamster chromosome 5, which gave rise to the CHO Z6, lacks an NOR and the Zl now has one. These data allowed regional assignment of ESD to the distal portion of Chinese hamster chromosome 1p and provided genetic evidence for the origin of CHO chromosomes Z1and Z6 from Chinese hamster chromosomes 1 and 5. Induced electrophoretic shift mutations of ESD and positive Ag-NOR staining for the rRNA genes on the Z1 showed that the activities of the genes lying close to the translocation breakpoints were maintained.