PRimed IN Situ labelling (PRINS) is a highly specific and sensitive technique for detecting DNA sequences on human chromosomes in situ. PRINS is currently being introduced for research and routine analysis in clinical and cancer genetics. In this paper, we report a rational PRINS procedure for the rapid identification of marker chromosomes. Using this method it is possible to test a sample from a patient with up to eight different primers simultaneously on one slide. We have synthesized oligonucleotide primers that can differentially tag the human chromosomes, and with the protocol presented in this report we are able to identify the chromosomal origin of a marker chromosome within 2 hours.