1. The lipid compositions of the low-density lipoproteins, the high-density lipoproteins and the ultracentrifugal residue of human serum are presented, with emphasis on certain lipoprotein classes and lipid components not previously described. 2. Except for the lipoproteins with the lowest and highest densities, there is a trend for stepwise successive increase or, respectively, decrease in the relative amounts of the main constituents of lipoproteins. 3. High-density lipoprotein-2 and high-density lipoprotein-3 have different amounts of certain lipids; high-density lipoprotein-2 has relatively more free cholesterol and sphingomyelin; high-density lipoprotein-3 has more free fatty acids, diglycerides and ceramide monohexosides. 4. All the lipoproteins contain hydrocarbons of the alkane series. The greatest amount, which averages 4.4% of total lipid extracted, is in the ultracentrifugal residue; n-alkanes comprise 18-50% of the hydrocarbons. 5. All the lipoproteins contain ceramide monohexosides. The highest relative contents of these glycolipids are in high-density lipoprotein-3 and in the ultracentrifugal residue. 6. The ultracentrifugal residue contains 55% of the total quantity of free fatty acids present in serum. The remaining free fatty acids are distributed among the other lipoprotein classes. 7. The choline-containing phospholipids (phosphatidylcholine, lysophosphatidylcholine and sphingomyelin) comprise about 90% of the phospholipids in all the lipoprotein classes except the low-density lipoprotein-2, which contains about 80% of these phospholipids. 8. The presence of a large amount of lysophosphatidylcholine in the ultracentrifugal residue and the successive decrease of sphingomyelin from the low-density lipoprotein-1 to the ultracentrifugal residue was confirmed. 9. The low-density lipoprotein-2 and the ultracentrifugal residue are characterized by relatively high contents of the lower glycerides.

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In the course of studies of the lipid composition of human serum lipoproteins (1,2 ) we observed a new lipid in thin-layer chmmatograms (TLC) ( 3 ) of lipid extracts from high-density lipoproteins (HDL) from cancer patients. Similar extracts from HDL of normal subjects did not reveal this lipid. We since found that lipids extracted from transplanted animal tumors also contain a similar lipid.This preliminary communication describes the general chemistry of this lipid, its distribution in rat tissues, its presence in sera from cancer patients, and implications of these observations. Nature of new lipid. The new lipid was isolated from lipid extracts of Walker carcinosarcoma 2 5 6 (W256) by silicic acid column chromatography with stepwise gradient elution (chloroform-methanol mixtures). The isolated lipid had the same Rf in several TLC systems (see below) as did the new lipid in the sera HDL of cancer patients. Complete drying of the isolated new lipid from W 2 5 6 in vacua (with N2 bubbling) caused partial decomposition. Decomposition products, identified by TLC (3, 4 ) , included phosphorus and nonphosphorus containing lipids. It was inferred that the new lipid is a complex unstable under certain conditions. In addition to complete drying, contact with sodium oxalate and treatment with ultrasonic vibration also cause decomposition of this complex. Further chemical studies of the complex revealed that protein (or polypep-1This researoh project was supported in