Myeloperoxidase (MPO) is an enzyme whose synthesis is restricted to the promyelocytic stage of myeloid differentiation. We have recently described the cloning and sequencing of a cDNA for MPO. Using a regional mapping panel of somatic cell hybrids containing various deleted or translocated segments of chromosome 17, we have assigned MPO to a region between 17q21 and 17q23. In situ hybridization refined this localization in that grains on chromosome 17 were significantly clustered at bands q22-23 and no hybridization was detected at q21. In light of this chromosome assignment, the relationship of MPO to the 17q translocation breakpoint characteristic of acute promyelocytic leukemia (APL) was considered. Because this breakpoint has been variously assigned to different bands on 17q from 17q11.2 to 17q22, the cytogenetic and molecular distance between this breakpoint and MPO cannot be accurately determined. MPO and other probes mapped to this region of 17 will be important in searching for altered Southern blot patterns after conventional or pulsed-field gel analysis of DNA from APL patients.