Oxidant injury and release of proteolytic enzymes in prematures with respiratory distress syndrome (RDS), who are treated with ventilators and oxygen, have been postulated as possible causes of bronchopulmonary dysplasia (BPD). The premature may be at particular risk due to low levels of antiproteases, such as alpha-1-proteinase inhibitor (alpha 1PI), and antioxidants, such as ceruloplasmin (CER). Both alpha 1PI and CER deficiencies have been correlated with the severity of RDS. We studied serial alpha 1PI activity as measured by trypsin inhibitory capacity (TIC) and CER in the serum 27 prematures who required ventilator therapy for RDS. Serum TIC values for day 1 were significantly lower (0.34 vs. 0.92 mg inhibited/ml of sample) in the 13 patients who developed BPD compared to the 14 who did not. No significant differences were seen on succeeding days. No significant differences in CER were seen, although both groups had levels 33-50% of adult normals (11.3 vs 9.3 mg/dl). Other significant variables included birthweight (p less than 0.005), severity of RDS (p less than 0.03), and gestational age (p less than 0.03). One way analysis of variances demonstrated day 1 TIC to be the most significant variable (p less than 0.0001), followed by weight (p less than 0.007), severity RDS (p less than 0.04), and gestational age (p less than 0.03). CER levels were not a significant variable. A formula utilizing unstandardized canonical discriminant function including day 1 TIC, birthweight, severity of RDS, and gestational age was 100% sensitive and 85% specific in the prediction of BPD for the original study group. In an additional 25 consecutive admissions with severe RDS of whom 18 survived, the formula was 100% sensitive (6/6) and 75% specific (9/12).(ABSTRACT TRUNCATED AT 250 WORDS)