The amount of Mn superoxide dismutase (MnSOD) and the activity of Cu,Zn-superoxide dismutase (CuZnSOD) have been studied in human fibroblasts of five subjects with trisomy 21 and five subjects with normal karyotype, using nuclear magnetic relaxation and polarographic methods. In the trisomic fibroblasts we have found a mean molar amount of MnSOD 25.4% lower than in the control, and an amount of CuZnSOD 54.7% higher. A positive significant correlation between the activities of both enzymes has been observed indicating that the two enzymes dismute the O2- cooperatively. However, the increase of MnSOD per unit of CuZnSOD appears significantly lower in the trisomic fibroblasts, an effect that is not due to a diminished inducibility of MnSOD. These findings suggest that the MnSOD and CuZnSOD genes interact to preserve the normal level of total SOD activity.