The transfer of insoluble cholesteryl esters among lipoprotein particles is a vital step in normal cholesterol homeostasis and may be involved in the development of atherosclerosis. Extrahepatic tissues lack the enzymes required for the degradation of sterols to the excretable form of bile acids. Cholesterol synthesized in these tissues in excess of that needed for the synthesis of cell membranes or steroid hormones must accordingly be returned through the plasma to the liver for catabolism. The series of reactions involved has been termed reverse cholesterol transport. Catalysed steps of this pathway are believed to include an efflux from peripheral cells, which generates a diffusion gradient between these membranes and extracellular fluid; esterification of this cholesterol by lecithin-cholesterol acyltransferase (LCAT) (phosphatidylcholine-sterol acyltransferase) acting on species of high-density lipoproteins; transfer of the cholesteryl esters formed (largely to low- and very low-density lipoproteins) (LDL and VLDL) by a cholesteryl ester transfer protein (CETP); and removal of these lipoproteins, together with their cholesteryl ester content, by the liver through receptor-mediated and nonspecific endocytosis. Of these steps, the CETP reaction is the least characterized. Several laboratories have reported the purification from human plasma of proteins active on cholesteryl ester transfer between lipoprotein particles and possibly between cells and plasma. However, the reported relative molecular mass (Mr), abundance and specificity of the purified activities have differed considerably. We have recently described the preparation of a highly active CETP of Mr 74,000 purified about 100,000-fold from human plasma, which may represent the functional component of earlier preparations. Using a partial amino-acid sequence from this purified protein, CETP complementary DNA derived from human liver DNA has been cloned and sequenced and the cloned DNA used to detect CETP messenger RNA in a number of human tissues.