Defibrotide (D) is an antithrombotic and profibrinolytic drug which is prepared from mammalian lung DNA. Studies using radiolabeled defibrotide demonstrated that considerable amount of the drug goes to the nuclei of cultured endothelial cells. Since the drug goes to the nuclei, it may exert its effects by induction of gene expression of certain proteins and/or by influencing cell cycle progression. In this study, we used flow cytometric techniques to estimate the effects of defibrotide on cell cycle parameters of cultured endothelial cells. No statistical difference in the fraction of cells at G,, S, G2 +M phases was observed between control cells and the cells treated with defibrotide both at different times and confluency levels.