Serum creatine kinase activity was measured during the first post-natal days in healthy full-term and premature infants. The CK isoenzymes (CK-MM, MB and BB) were separated using ion-exchange column chromatography. Total CK activity is lower for premature infants than for full-term infants at the same time-periods. However the separation of the CK isoenzymes shows that the same normal values for the CK-BB (expressed as U/l) may be used for the two groups of infants.