We have investigated the use of the satellite III probe, D15Z1, as an alternative to DA/DAPI staining in the identification of chromosome 15-derived markers. The probe hybridises to the short arm of chromosome 15 under high stringency conditions. We have screened 100 randomly selected patients, by fluorescence in-situ hybridisation (FISH), using co-hybridisation experiments with D15Z1 and a whole chromosome library, pBS-15. 88 individuals showed the expected pattern of two D15Z1 signals on the p-arm of both homologues of chromosome 15, whereas 12 individuals showed an additional signal on a third acrocentric D-group chromosome. Sequential GTC banding and D15Z1 hybridisation revealed that in each case it was one homologue of chromosome 14 that was D15Z1 positive. This pattern always correlated with positive DA/DAPI staining. In contrast, the 15 centromere-specific alphoid probe, pTRA-25, gave the expected two signals on both homologues of chromosome 15 in every case. Thus, D15Z1 and DA/DAPI signals co-localize while pTRA-25 is 15 centromere-specific and should be applied for unequivocal identification of chromosome 15-derived markers in clinical studies. The chromosome 14 heteromorphism, as identified by D15Z1, and defined by pTRA-25, may have arisen by intrachromosomal amplification or interchromosomal exchange.