Summary The location of human superoxide dismutase has been identified on 2-D gels of cultured fibroblast extracts. Human superoxide dismutase was located on 2-D gels of a mouse-human somatic cell hybrid containing only human chromosome 21, and was absent in the mouse parental cell line. The native enzyme is a diner composed of two identical monomers present on 2-D gels as a single spot of molecular weight approximately 16,000 and isoelectric point of pH 6.2. Comparison of the density of the SOD spot on gels of a trisomy 21 cell strain to that of a monosomy 21 cell strain showed a 3 fold increase indicating agene dosage effect was present. This approach demonstrates the utility of quantitative 2-0 analysis for gene dosage studies.