Five different apolipoproteins (apo) and lipid profiles were studied in breast-fed mature (BM), brest-fed premature (BP), formula-fed mature (FM) and formula-fed premature (FP) infants in the first year of life. The plasma apo A-1, B, C-II and C-III levels in all groups of infants increased rapidly after birth until 4 weeks of age, and then became stable. On the other hand, apo A-II levels did not change markedly in any of the groups of infants during the neonatal period, but there was a gradual increase in mature infants thereafter. At 12 months of age, apo A-I, A-II and B remained significantly lower than in adults. There were no significant differences in any of the apolipoprotein levels between BM and FM infants or between BP and FP infants except for apo A-I and C-III of premature infants at 3 weeks of age. When premature and mature infants were compared, premature infants showed lower apolipoprotein levels except for apo A-II, and the differences were especially prominent in the neonatal period. Plasma triglyceride levels rapidly increased in the neonatal period and were stable thereafter. Whereas plasma total cholesterol levels increased gradually until one year of age. There were no differences between breast-fed and formula-fed infants. Both triglyceride and total cholesterol levels in premature infants were lower than those in mature infants during the neonatal period. Thus, the proportional change in plasma lipids and apolipoprotein levels during this study period may indicate that lipoprotein composition is not markedly deteriorated even in different nutritional condition in the first year of life.