Most of the cholesterol in intestinal chyle and chylomicrons is derived from plasma. Our aim was to determine how much plasma low density (LDL) and high density (HDL) lipoproteins contribute to the cholesterol in chyle and chylomicrons, and to examine how plasma cholesterol becomes associated with lymph chylomicrons. Intravenous injection of radioiodinated plasma lipoproteins into two chyluric patients showed that 82% of the HDL plasma pool transferred daily to intestinal chyle, corresponding to 58% of lymph cholesterol; LDL contributed 18% of its plasma pool, corresponding to 18% of lymph cholesterol. When plasma HDL radiolabeled in both the protein and cholesteryl ester moieties was injected, the isotope ratios of plasma HDL and lymph lipoproteins were identical; 85% of the HDL cholesteryl esters transferred to triglyceride-rich lipoproteins, while the apolipoproteins remained largely (70%) in the higher density lipoproteins of the chyle. Incubations of similarly labeled plasma HDL showed preferential transfer of cholesteryl esters to artificial chylomicrons mediated by a factor present in lipoprotein-free plasma. Thus, a sizable portion of plasma HDL enters intestinal lymphatics probably as intact HDL, and then transfers part of their cholesteryl esters to chylomicrons, possibly mediated by transfer proteins. Reverse cholesterol transport may therefore include an extravascular loop via lymph chylomicrons and chylomicron remnants to the liver.