Oligonucleotide probes have been used to map the myeloperoxidase (MPO) gene locus to chromosome bands 17q21-22. This is in agreement with results reported using conventional cDNA probes. No evidence for the existence of a second MPO gene locus was obtained. Six synthetic 72-base oligonucleotides, corresponding to different exon regions of the MPO gene, were tritium-labeled and used as in situ hybridization probes. Synthetic oligonucleotide probes offer a useful alternative to conventional DNA probes for gene mapping.