Water-insoluble lipids circulate in serum bound to specific proteins as a mixture of soluble macromolecular lipoprotein complexes3 that, over the full density range, are characterized by gradually changing physicalchemical properties such as size, density, and lipid composition and by differences in the nature of the protein moieties (apolipoproteins). 1* 3 According to present concepts, nearly all long-chain dietary triglycerides absorbed by the intestinal mucosa, as a mixture of fatty acids and monoglycerides, are transported to serum by way of the thoracic duct lymph. 4s6 Approximately 95% of the exogenous fatty acids are transported as resynthesized triglycerides, and only negligible amounts as phospholipids (three to four percent) and cholesterol esters (one to two per~ ent).~~'Triglycerides and cholesterol esters combine with phospholipids, free cholesterol, and proteins, most probably in the intercellular spaces of the epithelium cells, and enter the lymphatic channels in the form of chylomicrons and other lipoproteins. 8 With the exception of chylomicrons, virtually nothing is known about the chemical composition and physical-chemical properties of human chyle lipoproteins. It has been surmised9J0 that they are not formed like chylomicrons in the intestinal cells but are derived mainly from the liver lymph. However, in Vitro experiments with mucosal cells isolated from the dogll and intestinal slices from the rat12 have shown that radioactive amino acids could be incorporated into the protein moieties of both chylomicrons and higher-density lipoproteins. It has been found13 that, in the anesthetized cat, liver lymph contributes about 30% of the total thoracic duct lymph volume and approximately 40% of the total protein, the remaining thoracic duct lymph being derived almost entirely from the intestinal tract. Furthermore, it has been shown, in the same study, 13 that only the intestinal lymph fatty acids, phospholipidi and cholesterol show large increases during fat