 The cholesteryl ester transfer protein activity was measured by an endogenous assay. Aliquots of the whole plasma in which LCAT activity was inhibited by DTNB 9 μLmL were added to HDL-3Hcholesteryl ester fractions and simultaneously incubated at 4C and 37C for 4h. Apo-B containing lipoproteins, present in the incubation mixture, were then precipitated; the CE radioactivity in the supernatant represented the net rate at which CE mass was transferred and values expressed as percent of 3H cholesteryl ester transferred4 hours depended upon the plasma concentrations of HDL, TG-rich lipoproteins and CETP simultaneously. 