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Lipids, apolipoproteins, lipoproteins, as well as lipoproteins containing both apo A-I and apo A-II (Lp A-I:A-II) or apo A-I but no apo A-II (Lp A-I), proapolipoprotein (proapo) A-I and the activity of lecithin:cholesterol acyltransferase (LCAT), were investigated in umbilical cord sera of 67 term human neonates (30 females and 37 males). Lp A-I and Lp A-LA-II were present in umbilical cord sera with levels of 0.26 ± 0.1 and 0.33 ± 0.15 g/l, respectively. Furthermore, the absolute amount of proapo A-I was lower in cord blood than in adult plasma, but in view of the lower apo A-I levels in umbilical cord sera it comprised 10.48 ± 3.86% of total apo A-I and was thus significantly higher than in adult plasma (7.1 ± 0.9%). Proapo A-I was highly correlated with HDL cholesterol and apo A-I. Total serum LCAT activity was about 50% of adult plasma and was highly correlated with Lp A-I, but not with Lp A-I:A-II. We conclude that human umbilical cord serum contains both Lp A-I and Lp A-I:A-II particles and that the LCAT activity is predominantly related with the Lp A-I subfraction. The higher percentage in umbilical cord sera of proapo A-I may indicate a higher turnover of apo A-I or a lower activity of the proapo A-I cleaving enzyme which is still not identified.