The parallelism between the human nucleolus organizer region (NOR) activity and the stability of human satellite chromosomes in Chinese hamster-human somatic cell hybrids cultivated in media containing 5-azacytidine (5-AzaC), a demethylating agent, have been investigated. The interspecific hybrids showed no active NORs on human satellite chromosomes even at early stage after hybridization. Moreover, human chromosomes including satellite chromosomes were considerably segregated after successive passages in the hybrids. On the contrary, the hybrids showed an average 2.5 active human NORs and the stable retention of human chromosomes even 40 passages after hybridization when they were successively exposed to 5-AzaC. These results suggest that demethylation of human NOR may result in delayed segregation or stable maintenance of human chromosomes through the reactivation of NOR activity in the interspecific hybrids.