ABSTRACT:Disagreement as to whether Ig levels are high, low, or normal at different ages in different groups of trisomic children has interfered with defining the associated immunodeficiency state. Fetal Ig production was assessed by measuring cord blood IgM and IgA of five trisomy 21, 97 normal, and 37 control newborns with other birth defects. Trisomic infants showed significantly lower values. Cord blood IgG values showed no differences between groups when corrected for fetal birth weight. The data indicate that children with trisomy 21 have an impaired humoral immune response in utero.