Chinese hamster × human and mouse × human somatic cell hybrid lines were obtained using circulating leucocytes from six chronic myeloid leukemia patients. All six patients carried the Ph1 translocation, t(9q+;22q–), characteristic of chronic myeloid leukemia, in their dividing immature granulocytes. Analysis of independent hybrid clones yielded the following results: 1. The chromosome 9 markers, soluble aconitase and adenylate kinase-1, segregated with the 9q+ derivative. The latter marker has previously been localized to 9q34. 2. The chromosome 22 markers, mitochondrial aconitase, N-acetyl-α-D-galactosaminidase, and arylsulfatase-A, also segregated with the 9q+ derivative. Mitochondrial aconitase has recently been assigned to 22q11→22q13. No evidence was obtained either for reciprocity of the translocation or for variations in breakpoints in different patients. The results reported in this paper provisionally assign the gene for mitochondrial aconitase to a region distal to the breakpoint in