SummarySerum factors related to oxygen exposure were studied in 56 full-term cord blood samples and in 69 newborn infants of varying gestational age (GA). Serum malondialdehyde (MDA), which reflects membrane lipid peroxidation, was elevated during the first 2 d of life and rose to a peak at 3-5 d of life. This peak value was unrelated to GA or to assisted ventilation.The serum antioxidant, vitamin E, showed a significant rise by 6-10 d, and came into the adult range after d 11. Vitamin E levels did not correlate with GA, assisted ventilation, or the development of bronchopulmonary dysplasia (BPD). Serum ceruloplasmin, another antioxidant, was measured both by activity assay and by protein concentration assay. Little activity was found in cord blood. Ceruloplasmin activity increased during the first 48 h of life, and both activity and protein concentration correlated with GA at that time. Infants who subsequently developed BPD had a less active protein than infants on ventilators who did not develop BPD. In addition, activity and protein levels on 3-5 d were lower in infants on ventilators than in those not requiring assisted ventilation.Serum levels of a-l-AP activity and protein concentration were also correlated with GA during the first 48 h of life. The less mature infants had levels of activity and protein which were significantly less than the more mature infants and significantly less than the full-term cord values. The proportion of active protein correlated with GA at 3-5 d, indicating that the less mature infants had a lower proportion of active protein. All infants had activity and protein levels within the normal range for healthy adults by 6-10 d.