Blood obtained from umbilical cords of 106 preterm infants was analyzed for carbonic anhydrase (CA) activity. Carbonic anhydrase isoenzymes were measured in red blood cells from five adults, eight full-term newborn infants, and 16 premature infants. Carbonic anhydrase activity increased progressively with gestation. Infants with respiratory distress syndrome (RDS) had lower cord blood CA activity than infants without RDS. Carbonic anhydrase B (CA-B), carbonic anhydrase C (CA-C), and total carbonic anhydrase protein (CA-B + CA-C) were highest in adults, then full-term newborns, premature newborns without RDS, and infants with RDS. The ratio of CA-B/CA-C was the same for all newborn groups and lower than the adult ratio. All enzyme activity in newborns could be accounted for by CA-B and CA-C. No inhibitors or abnormal enzymes were present. Importance of findings to RDS pathogenesis is discussed.