Lipoproteins in newborn plasma were isolated from a minimal sample amount (0.3 ml) by a single-step ultracentrifugation in a density gradient, spanning the density range 1.02–1.20 g/ml. After 66 h ultracentrifugation in a swinging-bucket rotor, the content of the tube was eluted and collected in 0.4 ml fractions. Cholesterol and apoproteins AI, AII, and B were assayed in each fraction yielding both the distribution and composition of the very low density lipoprotein (VLDL), low density lipoprotein (LDL), high density lipoprotein (HDL2, and HDL3). Newborn plasma was characterized by a low amount of triglyceride-poor and cholesterol ester-rich VLDL and high content of HDL2and HDL3. The VLDL and LDL concentrations increased drastically between 0 and 7 days together with the triglyceride content of the VLDL.At 30 days the lipid composition of VLDL was similar to that of adults, whereas the lipid/protein content remained low both in VLDL and LDL. The composition of HDL2and HDL3remained constant during this period, the percentage of HDL2being higher in newborns than in adults. These compositional changes were reflected in the microviscosity of the lipoproteins, specially in the VLDL fraction.