Quantitative red cell adenylate kinase (AK-1) assay has been used in 8 patients with partial duplication or deletion of chromosome 9 in an attempt to find the precise intrachromosomal location of the structural gene locus. All regions of chromosome 9 are represented in abnormal dosage in at least one patient. A 43% increase in AK-1 activity was found to be associated with duplication of the terminal band of the long arm of chromosome 9. Duplication of all other parts of chromosome 9 were associated with normal enzyme activity. These findings not only confirm the assignment of the AK-1 locus to chromosome 9 made previously in somatic cell hybrids, but suggest a more precise assignment to region 9q33 leads to qter. This places the ABO:Np-1:AK-1 linkage group at the distal end of the long arm of chromosome 9.