Copper superoxide dismutase (Cu-SOD or SOD-1) was measured by radioimmunoassay (RIA) in amniotic cells and fluid as well as in fetal trisomy 21 fibroblasts. The significant increase of the SOD-1 activity observed in amniotic cells and fetal trisomy 21 fibroblasts demonstrates the early physiopathogenic expression of the supernumerary chromosome 21 in affected patients.