The centromeric alpha satellite DNA subfamilies from chromosomes 13 and 21 are almost identical in sequence and cannot be easily distinguished by mean of probes for Southern blot or in situ hybridisation. We have used the oligonucleotide-primed in situ (PRINS) labelling technique with primers defined from the alpha satellite sequence of chromosome 13. One primer was found to label specifically the centromeric region of chromosomes 13 and allowed the detection of a polymorphism between two chromosome 13 homologues in one individual.