Objective To investigate the effects of different dosages of bone marrow mesenchymal stromal cells (BMSC) on lung fibrosis. Methods BMSCs with red fluorescence protein (RFP) from male FVB mice were cultured in vitro. Twenty-four female wild type FVB mice were randomly divided into four groups: normal group, model group, BMSC 1 group and BMSC 2 group (n=6). Mouse pulmonary fibrosis models were induced by bleomycin via single intratracheal perfusion. Twenty-four h after model establishment, mice in BMSC 1 group and BMSC 2 group were injected with 1×106 BMSCs and 2×106 BMSCs, respectively through vena caudalis for each mouse. All the animals were sacrificed 21 d after model estalishment, and mouse lung tissue samples were obtained. The pathological changes were observed by light microscopy, the hydroxyproline (Hyp) contents were measured by alkaline hydrolysis assay, the distribution of RFP(+) BMSCs and quantitation of RFP were analysed by laser scanning confocal microscopy and immunohistochemistry, the expression of surfactant protein A (SP-A) was detected by immunohistochemistry, and the expression of transforming growth factor-β (TGF-β) and platelet-derived growth factor (PDGF) mRNA was detected by Real-time PCR. Results Compared with model group, the pulmonary fibrosis in BMSC 1 group was significantly alleviated, and that of BMSC 2 group became much more severe. A large number of RFP(+) BMSCs were found in fibrosis area of BMSC 2 group, which exhibited morphology similar to fibroblasts. As far as the expression of SP-A was concerned, normal group was higher than BMSC 1 group, BMSC 1 group was higher than BMSC 2 group and model group (P<0.05), while there was no significant difference between BMSC 2 group and model group (P＞0.05). Normal group, BMSC 1 group, model group and BMSC 2 group fell in the increase order by Hyp contents (P<0.01, P<0.05), and BMSC 2 group, BMSC 1 group, model group and normal group fell in the decrease order by expression of TGF-β and PDGF mRNA (P <0.05）. Conclusion Proper dose of BMSC has a favourable effect on bleomycin-induced lung fibrosis, while excessive dose of BMSC can aggravate the fibrosis.