Abstract The lipoproteins of human serum have been separated into 4 fractions by the use of differential centrifugation. Each fraction has been analyzed for free and total cholesterol, lipid phosphorus, total lipid, and protein nitrogen. From these data the composition of the lipoprotein fractions has been calculated. All fractions contain all these elements but in differing proportions. Thus chylomicrons contain about 81 per cent triglyceride, but even the highest density lipoproteins may contain up to 8 per cent triglyceride. The protein content varies from 2 per cent in chylomicrons to 47 per cent in the high density lipoproteins. The ratio of free to total cholesterol is highest in chylomicrons and lowest in the high density molecules.