Erythrocyte carbonic anhydrase activity was measured in 7 adults, 12 mature newborn infants, 12 preterm low birthweight infants, 9 `dysmature' infants, 33 infants with the respiratory distress syndrome (RDS), and 5 infants who had received intrauterine transfusions. The mean level of enzyme activity in infants with RDS was 3·6% of normal adult level, 21% of the mean level in mature infants, and 30% of the mean level in dysmature infants. There was, however, no significant difference between the mean enzyme activity found in infants with RDS and in low birthweight, preterm infants without RDS. In 3 of the 5 infants treated by intrauterine transfusion RDS developed while the carbonic anhydrase activity was up to adult levels. It is unlikely that the very low enzyme levels found in premature infants have any aetiological relation to the development of RDS, and there is no indication for treatment by exchange transfusion.