Effects of Intralipid on the increase of esterified cholesterol (EC) in each lipoprotein fraction in the fresh fasting plasma (FP) or triglyceride lipaserich plasma (PHP) from normal, and type V hyperlipoproteinemic subjects were investigated. FP or PHP was mixed with ¹⁴C-FC-albumin solution and was preincubated at 4°C for two hours. The mixture was incubated with Intralipid or with 0.15M NaCl solution. Incubation mixtures were collected at 0, 120 and 240 minutes. Chylomicron, VLDL, IDL, LDL and HDL were fractioned ultra-centrifugally at 4°C. Lipids were extracted with chloroform-methanol (2:1) and EC and FC were separated by TLC. The radioactivities of them were measured with a liquid scintillation counter. In the normal FP, Intralipid significantly increased ¹⁴C-EC in chylomicron, and VLDL fractions. However, Intralipid reduced cholesterol esterification, and this was solely due to the complete suppression of ¹⁴C-EC increase in HDL in normal PHP. The specific activities of EC in HDL were significantly lower than those in LDL in normal PHP with Intralipid. In FP of type V hyperlipoproteinemia, the increase of ¹⁴C-EC was more in chylomicron and VLDL fractions and less in LDL and HDL fractions than that in normal FP. The effect of Intralipid addition was very small in FP of type V hyperlipoproteinemia. In PHP of type V hyperlipoproteinemia, the results were very similar to those in normal FP with Intralipid. No increase of ¹⁴C-EC was observed in HDL. The effect of Intralipid addision was also small as in FP. Therefore, we concluded as following. Intralipid worked like chylomicron. There were two pathways for cholesterol esterification in the plasma. One was active in HDL and was blocked almost completely by Intralipid or chylomicron during the lipolysis. The other was active in other lipoproteins and was not inhibited by Intralipid or chylomicron during lipolysis.