Creatine kinase (CK) isoenzymes are considered to be specific tissue injury markers. The purpose of this study was to evaluate CK isoenzymes (CK-MM, CK-MB, and CK-BB) in umbilical cord blood sera of newborns in relation to their acid-base status. Investigations were performed in a group of 75 newborns delivered after 37 weeks of pregnancy. We estimated CK-isoenzymes with the use of DEAE-Sepharose C1-6B column chromatography. Total CK activity was measured using kits supplied by Boehringer-Mannheim (Monotest-R CK-NAC aktiviert). Newborns were considered hypoxic if umbilical artery blood pH was under 7.2 and/or base excess less than -10 mmol/L. Of 75 newly born infants 26 had features of perinatal hypoxia. We were unable to demonstrate significant differences in total CK, CK-MM and CK-MB activities in examined groups of newborns. We found a significant rise of CK-BB activity in cord sera of hypoxic infants. Our results show that the fetal brain is the most vulnerable to a hypoxic state.