The effect of apolipoprotein (apo) composition of high density lipoproteins (HDL) on cholesteryl ester transfer protein (CETP) activity was studied by measuring the rate of radiolabeled cholesteryl esters transferred between low density lipoproteins (LDL) and HDL3 which contained various proportions of apoAI and apoAII. Ultracentrifugally isolated HDL3, which contained virtually only apoAI and apoAII in their protein moiety, were progressively enriched with apoAII upon the incubation with increasing amounts of delipidated HDL apolipoproteins. The substitution of apoAII for apoAI in HDL3 did not induce marked alteration of the lipid composition of the lipoprotein particles. The rates of cholesteryl ester exchanges with LDL in the presence of purified human CETP were significantly reduced with apoAII-enriched HDL3 as compared with non-enriched homologous particles. Consistent results were obtained by determining the rate of cholesteryl esters transferred either from LDL toward HDL3, or in the opposite direction, from HDL3 to LDL. The effect of the apoAI and apoAII content of HDL particles on CETP activity was also investigated by measuring the rate of cholesteryl esters transferred from LDL to plasma HDL3 particles which contained either only apoAI, HDL3-AI, or both apoAI and apoAII, HDL3-AIAII. HDL3-AI and HDL3-AIAII particles were isolated from human plasma by a sequential procedure which combined ultracentrifugation and anti-apoAII immunoaffinity chromatography. As observed with HDL3 artificially enriched with apoAII, cholesteryl ester transfer rates were significantly lower with plasma HDL3-AIAII than with plasma HDL3-AI particles. Kinetic analysis of the interaction of CETP with apoAII-enriched HDL3 revealed that apoAII could act as an uncompetitive inhibitor of the cholesteryl ester transfer reaction. Since the plasma levels of HDL-AI, HDL-AIAII, and HDL-AII may undergo significant physiological fluctuation, the present study suggests that HDL apoproteins may be important factors in modulating cholesteryl ester transfer rates in vivo.