['To explore the mechanisms of endothelial progenitor cells (EPCs)on promoting osteogenic differentiation of marrow stromal cells (MSCs).', "EPCs and MSCs were isolated and cultured successfully from C57BL/7 murine bone marrow by in vitro amplification. EPC-conditioned medium (CM) was extracted to detect the concentrations of vascular endothelial growth factor (VEGF), transforming growth factor-beta 1 (TGFβ1), platelet-derived growth factor (PDGF), insulin-like growth factor 1 (IGF-1), stromal cell-derived factor 1 (SDF-1) and basic fibroblast growth factor (bFGF) by enzyme-linked immunosorbent assay (ELISA). After 14 days of induction, alizarin red staining was used to detect every group's calcium salt deposition. And analyses were conducted for the effects of above mentioned antibodies of cytokines on osteogenic differentiation of MSCs.", 'Positive rates of EPCs were 79.3%, 79.5%, 76.4% for VEGFR2, CD34 and CD133 respectively, and EPCs could form tube-like structure on matrigel. EPCs secreted VEGF, TGFβ1, PDGF, IGF-1, SDF-1 and bFGF. MSC/EPC group formed more mineralized nodules than MSC group, and semi-quantitative results showed the optical density of MSC/EPC group was higher than that of MSC group (0.733 ± 0.032 vs 0.236 ± 0.020, P < 0.001). The number of formed mineralized nodules of 50% EPC-CM group were more than those of 25% EPC-CM group, and semi-quantitative results showed that the optical density of 50% EPC-CM group was higher than that of 25% EPC-CM group (0.637 ± 0.028 vs 0.336 ± 0.024, P < 0.001), the number of formed mineralized nodules of 25% EPC-CM group were more than those of 0 EPC-CM group, and semi-quantitative results showed that the optical density of 25% EPC-CM group was higher than that of 0 EPC-CM group (0.336 ± 0.024 vs 0.239 ± 0.013, P = 0.004). On the basis of 50% of EPC-CM, the number of formed mineralized nodules significantly declined in the presence of anti-VEGF antibody, anti-TGFβ1 antibody, anti-IGF-1 antibody (P < 0.01).'] Do [ Endothelial progenitor cells promote osteogenic differentiation of marrow stromal cells in a paracrine manner ]?