Cholesteryl ester transfer protein (CETP) activity was measured ind > 1.21 g/ml plasma from hypertriglyceridemic patients and compared with normolipidemic subjects. The assay consisted in measuring the specific transfer of [ 3 H]cholesteryl oleate from a prelabelled, apo E-poor HDL fraction to VLDL after incubation at 37°C in the presence of the d > 1.21 g/ml plasma sample: the lipoproteins were then separated by precipitation with dextran sulfate/Mg2+ solution. Increasing the volume of d > 1.21 g/ml plasma or purified human CETP in the assay produced linear responses in measured activity, whereas, either during incubation at 4°C or in the presence of rat plasma instead of human plasma, the transfer of [ 3 H]cholesteryl oleate to VLDL was not stimulated. Thus, the assay reflects changes in CETP in the sample and appears to be suitable for measuring CETP activity in d > 1.21 g/ml plasma. CETP activity was very similar in the two groups of normolipidemic subjects considered: adolescents (203 ± 11 nmol esterified cholesterol transferred per 8 h/ml plasma) and adults (215 ± 5). Patients were grouped into lipoprotein-lipase (LPL)-deficient and non-LPLdeficient according to their enzyme activity in postheparin plasma. CETP activity was highly increased in LPL-deficient, severe hyperchylomicronemic patients (430 ± 42) and was directly correlated with VLDL levels in the non-LPL-deficient individuals. Marked differences were observed in the lipid composition of HDL and apolipoprotein A-I levels among patients and controls. In the control group, CETP activity was correlated only with HDL-triglyceride and HDL-triglyceride/apo A-I mass ratio, which is compatible with the physiological role of CETP in transferring triglyceride to HDL from other lipoprotein particles. When all hypertriglyceridemic patients wen: considered together, CETP activity was inversely correlated with apo A-I and HDL-cholesterol, whereas it was directly correlated with HDL-triglyceride/HDLcholesterol and HDL-triglyceride/apo A-I mass ratios. The results indicate that the enhanced CETP activity associated with hypertriglyceridemia contributes to the compositional change of HDL, which in turn may be responsible for the reduction of HDL levels in this condition.