Traditionally the leaves of Oenothera rosea "chupasangre" are used to reduce the bruising in humans. Therefore, this research was focused at evaluating of the fractions capable of retarding blood clotting. From the ethanolic extract of O. rosea separated by thin layer chromatography on cellulose using ethanol: water (1: 5), F-2 and F-5, wich were preincubated for 10 minutes with thrombin and the venom of the Lachesis muta snake rich in EST (Thrombin-like enzyme) and then coagulant activity was measured on citrated human plasma and bovine fibrinogen, as well as the BApNA chromogenic substrate. The results showed that F-2 lengthened the clotting time over Fb 58,58 % and F-5 67,78 %, while using poison the delays were for F-2 10,67 % and F-5 36,27 %. Using plasma, the values were for F-2 34,14 % and F-5 70,59 %. Likewise, by using thrombin the amidolytic activity was reduced by F-2 48,48 % ...