In order to investigate the effects of bone marrow-derived MSCs (mesenchymal stem cells) in reversing liver fibrosis and to determine their possible mechanism of action, mouse MSCs were infused into the tail vein of a CCl(4) injection mouse chronic model. MSCs caused a decrease in liver fibrosis histopathologically, 4 weeks after transplantation. The reduction in liver collagen was confirmed by quantitative analysis. Moreover, lipid peroxidation in the CCl(4)/MSC group decreased significantly. Quantitative RT (reverse transcription)-PCR analysis showed administration of MSCs has a significant antifibrotic effect as evidenced by the decrease in expression of liver collagen and increase in MMP13 (matrix metalloproteinase 13) in the CCl(4)/MSC group when compared with the CCl(4) group, 4 weeks after transplantation. The expression of alphaSMA (smooth muscle actin) and TIMP1 was also down-regulated in the CCl(4)/MSC group. Additionally, the expression of MMP9 was significantly up-regulated in the CCl(4)-treated group; however, there was no significant change after MSC injection. Few engrafted cells in the recipient liver and were able to differentiate into albumin-positive cells. In conclusion, MSCs can enhance recovery of a CCl(4)-injured mouse liver through their influence in reducing collagen deposition by possibly affecting expression of MMPs.