Superoxide dismutase (SOD) activity was determined in the erythrocytes of 16 full-term and 12 preterm neonates and the mothers of these babies. Blood samples were obtained from the umbilical cord (or within the first 12 h and samples were again obtained 48 h after delivery. The results of the study show that SOD activity in the erythrocytes of the full-term newborn is identical to the SOD activity in the erythrocytes of their mothers. Exposing the newborn to atmospheric oxygen for 48 h caused no change in the activity of SOD. The activity of SOD in the erythrocytes of the preterm was not different from that of the full-term neonate.