SummaryWe measured blood glucose, serum insulin and apoprotein A-I and A-II, and triglycerides and cholesterol contained in serum lipoprotein fractions of 24 full-term newborn infants who underwent exchange transfusion with heparinized blood for hematological reasons. The values were similar to those previously reported for healthy newborn infants. We also measure lipoprotein and hepatic lipase activities with specific methods. Fifteen minutes after an intravenous heparin bolus of 100 IU/kg, mean lipoprotein lipase activity in infants (16.0 µmol free fatty acids/ml/h) was as in adults. In contrast, hepatic lipase activity was significantly higher in infants (54.3 µmol free fatty acids/ml/h) than in adults. There was no sex difference in the infant lipase activities. Lipoprotein and hepatic lipase activities were also measured 5 and 15 min after a heparin bolus of 10 and 50 IU/kg: 10 IU/kg released only part of the lipase activities. In addition, the two lipases were measured during the exchange transfusion. Although 92% of the original infant blood was removed, lipoprotein lipase activity remained constant. In contrast, hepatic lipase activity decreased considerably. In infants, postheparin lipolytic activity is a conventional measure of lipoprotein lipase. Lipoprotein and hepatic lipases comprise 95% of postheparin lipolytic activity. In our infants, hepatic lipase activity was 3.4 times higher than lipoprotein lipase activity. Hence, it seems unjustified to use postheparin lipolytic activity as a measure of lipoprotein lipase.