Abstract： Objective To investigate the effect of batroxobin (DF-521) on peripheral counts and in vitro capabilities of endothelial progenitor cells (EPCs) in patients with lower extremity deep venous thrombosis (LEDVT). Methods Patients with LEDVT were recruited and randomized into the control group (n=18) and the therapy group (n=18). The control group was given low-molecular-weight heparin (4000 U/12h)and urokinase (300 000 U/d),whereas the therapy group received DF-521 (5 U/2 d) in addition to the same dosages of heparin and urokinase as in the control group. The efficacy of two groups was evaluated at 7days and at 3 months after treatment. Before and on day 7 of treatment,blood samples were collected from patients and determined for the percentage of EPCs (CD34+ VEGFR2 + ) by fluorescence-activated cell sorting. From healthy blood samples,peripheral blood mononuclear (PBMN) cells were separated by density gradient centrifugation and induction-cultured into EPCs. The EPCs in vitro were divided into 4 experimental groups (incubated with various concentrations of DF-521:1 × 10-2,2× 10-2,4× 10-2,8× 10-2 U/ml) and a control group (culture medium only). The effects of DF-521 on proliferation,nitric oxide (NO) release,angiogenesis and differentiation of EPCs were observed by MTT assay,nitric oxide kits,tube formation assay,the phenotypes by flow cytometry,and cell ultrastrueture by transmitting electron microscopy (TEM).Results The treatment efficacy was statistically comparable on day 7 (66.7% vs 66.7%,P>0.05) but appeared better in the therapy group than that in the control group at 3 months (94.4% vs 66.7%,P<0.05).On day 7 of treatment,the therapy group showed an increase in peripheral EPCs percentage counts compared with baseline (0.64±0.05 vs 0.42±0.02,P<0.05) and with the control group (0.64±0.05 vs 0.42±0.03,P<0.05),while the control group did not show any change in EPCs count (0.41±0.03 vs 0.42±0.03,P>0.05).The absorbance (A value),NO level and number of lumen-like structure were greater in all experimental EPCs groups than those in the control group (P<0.05). Moreover,there was an increasing trend for A value and NO level along with higher level of DF-521 (r=0.978 and 0.981,both P<0.05). Incubation with 4×10-2U/ml DF-521 was associated with the greatest number of lumen-like structure identified. Compared with the control group,the expressions of CD31 and vWF were higher in all experimental groups (P<0.05) but appeared comparable between each other (P>0.05). TEM study found endothelium-specific Weibel Palade body in the group treated with 8×10-2 U/ml DF-521. Conclusion Treatment with DF-521 may significantly improve the prognosis of LEDVT,increase the peripheral EPCs count,increase the in vitro activities and capacities of EPCs,and promote differentiation of EPCs to endothelial cells.