Plasma high density lipoproteins (HDLs) from humans, from transgenic mice to human apolipoprotein A-I (HuAITg mice), from transgenic mice to human apolipoprotein A-II (HuAIITg mice), from transgenic mice to human apolipoproteins A-I and A-II (HuAIAIITg mice), and from C57BL/6 control mice were isolated, and their ability to interact with the human cholesteryl ester transfer protein (CETP) was studied. Whereas cholesteryl ester transfer rates were gradually enhanced by the addition of moderate amounts of HDL from the different sources, striking differences appeared when HDL levels kept increasing beyond a maximal transfer value. Indeed, while a plateau value corresponding to maximal CETP activity was maintained when raising the concentration of HuAITg HDL and HuAIAIITg HDL, inhibitions could be observed with the highest levels of human, control mouse, and HuAIITg mouse HDL. The concentration-dependent inhibition of CETP activity could be reproduced by the addition of delipidated HDL apolipoproteins from control mice, but it was abolished by a 1-h preheating treatment at 56°C. In contrast, no significant inhibition of CETP activity was observed with the delipidated protein moiety of HuAITg HDL, and cholesteryl ester transfer rates remained unchanged before and after a 1-h, 56°C preheating step. Finally, the CETPmediated transfer of radiolabeled cholesteryl esters from human low density lipoprotein to human HDL was significantly higher in the presence of lipoprotein-deficient plasma from HuAITg mice than in the presence of lipoprotein-deficient plasma from control mice. Interestingly, cholesteryl ester transfer rates measured with both control and HuAITg lipoprotein-deficient plasmas became remarkably similar following a 1-h, 56°C preheating treatment.It is concluded that human, control mouse, and HuAIITg mouse HDL contain a heat-labile lipid transfer inhibitory activity that is absent from HDL of HuAITg and HuAIAIITg mice. Alterations in CETP-lipoprotein binding did not account for differential lipid transfer inhibitory activities.In human plasma, the cholesteryl ester transfer protein (CETP) 1 promotes the exchange of cholesteryl esters and triglycerides between various lipoprotein fractions (1). In vivo, CETP activity results in the net mass transfer of cholesteryl esters from the antiatherogenic high density lipoproteins (HDLs) toward the proatherogenic apoB-containing lipoproteins, i.e. very low density lipoproteins and low density lipoproteins (LDLs) (1). The mechanism of action of human CETP has now been clearly established, and it involves two main steps. In a first step, positively charged amino acids of CETP can interact with negative charges of lipoprotein particles from which, in a second step, it picks up or deposits neutral lipid molecules (for a review, see Ref.2). The CETP-lipoprotein binding has been shown to constitute one major determinant of the CETPmediated neutral lipid transfer reaction (3), and both insufficient and excessive binding of CETP to lipoproteins resulting from alterations in th...