The implication of cholesteryl ester transfer protein (CETP) in the high density lipoprotein (HDL) conversion process was studied by incubating HDL3 with a purified CETP preparation for 24 h at 37 degrees C. At a physiological plasma level, CETP induced a decrease of the HDL3 fraction (8.6 nm diameter) and the appearance of two new distinct particle subpopulations with mean diameters of 9.5 and 7.8 nm. To determine whether the effects of the CETP preparation could be assigned to CETP itself, the incubations were conducted either in the absence or in the presence of specific anti-CETP monoclonal antibodies. The HDL3 conversion process induced by the CETP preparation was totally blocked by addition to the incubation mixture of TP1 anti-CETP monoclonal antibody, known to completely inhibit the cholesteryl ester transfer activity in vitro. Moreover, the HDL conversion activity was retained, together with the CETP activity, on an anti-CETP affinity column and was insensitive to the presence of a lecithin:cholesterol acyltransferase inhibitor. Compared with incubations with CETP, incubations with CETP and apoA-IV increased the size range redistribution of the HDL3 particles, particularly by promoting the formation of very small-sized lipoprotein particles. The results of the study demonstrate thhat CETP can mediate an HDL size conversion even in the absence of lipid transfers between HDL and other lipoprotein fractions. They constitute a supplementary argument for a multipotential role of CETP in lipid transport.