In a comparative study in term, preterm and low-birthweight infants, the mean activity and standard error of the mean for copper/zinc superoxide dismutase (Cu/Zn SOD) in cord erythrocytes from five term small for gestational age infants was 0.94 +/- 0.10 SOD units (mg protein)-1. This value was significantly lower than the activity (2.34 +/- 0.24) in nine term, appropriate for gestational age (AGA) babies (p < 0.005). In 15 preterm (AGA) infants, the activity at birth (1.05 +/- 0.07 SOD units (mg protein)-1) was also significantly lower (p < 0.001) relative to term AGA babies. An increased level of activity (1.59 +/- 0.09) was detected in the red cells of eight preterm AGA infants on their expected date of delivery compared with (0.87 +/- 0.06) at birth (p < 0.001). However, the activity (1.59 +/- 0.09) was still lower than that detected in term AGA babies (2.34 +/- 0.24; p < 0.02). Similar findings were obtained when enzymatic activity was expressed in units per millilitre of packed erythrocytes. The low activity of Cu/Zn SOD in preterm and low-birthweight babies may render them susceptible to diseases associated with membrane lipid peroxidation.