Aim To investigate the protective effect of apolipoprotein(apo) C Ⅱon the morphology and func- tion of human umbilical vein endcthelial cells,injured by low density lipoprotein (LDL) in vilro. Methods Cultured huynan endothelial cells derived from umbilical vein were divided into four groups:control, high density lipoprotein (HDL)-apo + LDL,apo C Ⅱ+LDL and LDL group. which were observed the morphological changes with phase-contrast and transmission electron microscope and measurcd the re- lease of lacate dehydrogenase (LDH) and level of 6-ke-to-prostaglandin Fla (PGFla).Results Endothelial cells after being injured by LDL showed cell contraction. Increased release of LDH ayld decreased jevel of 6-keto-PGFla. Howev- er, normal morphology and LDH rtlease as well as prostacyclin (PGI2) syntllesis in endothelial cells were found when HDL-apo or apo C Ⅱ was added to culture media before LDL inJury.Conclusious Both HDL-apo and apo C Ⅱ could re-sist the injurious effect of LDL on cultured endothelial cells.