Thrombin and low density lipoproteins (LDL) are capable of inducing damage to human endothelial cells in primary culture. These substances both induce endothelial contraction as first recognizable morphological change. However, the effect of thrombin appeared early, was reversible and prevented by hirudin, while LDL acted over hours and the injury was progressive. Addition and high density lipoprotein (HDL), together with LDL, inhibited the cellular injury induced by LDL.