The main objective of the present study was to develop an efficient and reliable probe to be routinely used for detection of chromosome 13 copy numbers by interphase FISH. To achieve this, a Yeast Artificial Chromosome (YAC) containing sequences specific for human 13q12 (744D11), was cultured and the whole yeast genomic DNA was extracted. The human insert within the isolated DNA was amplified by inter-Alu PCR reaction and the PCR product was used as probe to detect the copy number of chromosome 13 using FISH experiments on 150 uncultured amniotic fluid samples after labelling with biotin or digoxigenin. Using this method 97 percent or more of the randomly evaluated cells from uncultured blood samples and 93 percent of the uncultured amniocytes showed two distinct signals on each cell. The intensity of signals was comparable to those of chromosome-specific alphoid DNA probes. The results suggest that this approach can be reliably used for prenatal detection of chromosome 13 aneuploidy.