To study the protective effects of apolipoprotein AⅡ (apoAⅡ) on endothelial cells injured by low density lipoprotein (LDL) in vitro, human umbilical vein endothelial cells were cultured and divided into four groups: control, HDL+LDL, apoAⅡ+LDL and LDL group, which were observed the morphological changes with phase-contrast and transmission electron microscope and measured the release of lacate dehydrogenase (LDH) and level of 6-keto-prostaglandin F1α (PGF1α). The endothelial cells injured by LDL showed cell contraction, increased release of LDH and decreased level of 6-keto-PGF1α. However, normal morphology and LDH release as well as PGI₂ synthesis in endothelial cells were found when HDL or apoAⅡ was added to culture media before LDL injury. The results indicated that both HDL and apoAⅡ could resist the injurious effect of LDL on cultured endothelial cells.