Quantitative assays of soluble GOT were performed on fibroblasts (from two individuals) with duplications of the long arm of chromosome 10 (10q24 leads to qter). Improved karyologic technique demonstrated that the duplications differ by half a band. Correlation of GOTS activity with the karyologic data indicates that the structural locus for GOTS maps within band 10q24. This study demonstrates the utility of this approach to fine structural mapping of the human genome.