In a previous study, we showed that in situ injection of glycosaminoglycan mimetics called RGTAs® (ReGeneraTing Agents) enhanced neovascularization after skeletal muscular ischemia (Desgranges et al, FASEB, 1999). In the present study, we showed that the RGTA® OTR4120 modulated angiogenesis in the chicken embryo chorioallantoic membrane assay, in a dose-dependent manner. We therefore investigated the effect of OTR4120 on one of the most specific angiogenesis-regulating heparin-binding growth factors, vascular endothelial growth factor 165 (VEGF165). OTR4120 showed high-affinity binding to VEGF165 (Kd = 2.2 nM), as compared to heparin (Kd = 15 nM), and potentiated the affinity of VEGF165 for VEGF receptors –1 and 2 and for neuropilin-1. In vitro, OTR4120 potentiated VEGF165-induced proliferation and migration of human umbilical vein endothelial cells. in the in vivo MatrigelTM plug angiogenesis assay, OTR4120 in a concentration as low as 3 ng/ml caused a 6-fold increase in VEGF165–induced angiogenesis. Immunohistochemical staining showed a larger number of well-differentiated VEGFR-2-expressing-cells in MatrigelTM sections of OTR4120-treated plug than in control sections. These findings indicate that OTR4120 enhances the VEGF165-induced angiogenesis and therefor may hold promise for treating disorders characterized by deficient angiogenesis.