Objective To investigate the effect of nerve growth factor (NGF) on osteogenesis induced by bone morphogenetic protein-9 (BMP-9) in mouse embryonic fibroblasts (MEFs).Methods MEFs were respectively transfected with adenovirus-mediated NGF (NGF group), BMP-9 (BMP-9 group) and NGF + BMP-9 (combined group) and green fluorescence protein (GFP) (control group).Cytochemical staining was used to test the activity of alkaline phosphatase (ALP) 3 d and 5 d after treatment.Level of osteopontin (OPN) mRNA was detected by RT-PCR 9 d after treatment.Level of OPN protein was assayed by Western blot and immunocytochemistry 9 d after treatment.Mineralization was detected by Alizarin red staining 14 d after treatment.Results ALP activity in MEFs was elevated in BMP-9 group rather than in NGF group, but a significant increase in ALP activity was noted in combined group.In control group, BMP-9 group, NGF group and combined group, level of OPN mRNA was 0.92 ± 0.03, 1.28 ± 0.04, 0.94 ± 0.03 and 1.62 ± 0.04 respectively (F =214.60, P ＜ 0.01);level of OPN protein was 0.60 ± 0.05, 0.84 ± 0.03, 0.53 ± 0.05 and 1.27 ± 0.05 respectively (F =162.5, P ＜ 0.01).In comparison, OPN mRNA and protein were significantly up-regulated in combined group than in BMP-9 group (t =10.569 and 11.778,P ＜ 0.05).In control group, BMP-9 group, NGF group and combined group, relative density of OPN protein was 3.63 ±0.17, 6.27 ±0.30, 3.86 ±0.18 and 10.16 ±0.18respectively (F =602.6, P ＜ 0.01), with a significant higher level in combined group than in BMP-9 group (t =22.280, P ＜ 0.05).Level of mineralization was significantly higher in combined group than in BMP-9 or NGF group.Conclusion NGF can potentiate the osteogenesis induced by BMP-9 in MEFs.