We present the first documented NOR suppression in a hybridoma other than man-mouse for the hamster-chimpanzee hybrid cell line R48-26. Alu PCR and chromosome painting showed that in this cell line chimpanzee chromosomes 13-15-23 are maintained. NORs on chimpanzee chromosomes 15-23, whose presence was directly verified by FISH with H 28s rDNA, resulted inactive while telomeric rDNA on hamster chromosomes resulted active even if hamster chromosomes presented extensive rearrangements. We observed an all or nothing model in accordance with a model of regulation by selective transcriptional factors. The rearrangements of hamster chromosomes have not involved the location of NORs because they maintain a telomeric position.