Objective: To study the effect of mesenchymal stem cells (MSC) on experimental liver fibrosis in rats. Design and method: MSC were derived from bone marrow obtained from femoral and tibia] bones of male albino rats. MSC were separated, grown, and propagated in culture for 4 weeks and were characterized morphologically and by detection of CD29 by RT-PCR. They were then infused into the tail vein of female rats that received CC14 injection to induce liver fibrosis. Rats were divided into 4 groups: control, CC14, CC14 plus MSC, and MSC. Liver tissue was examined histopathologically and liver functions (ALT and serum albumin) were estimated for all groups. Y-chromosome gene (sry) was assessed by PCR in liver tissue of the female rats to confirm uptake of the male stem cells. Hydroxyproline content in liver tissue was assessed by chemical methods and expression of the collagen gene (type 1) was detected as a marker for liver fibrosis. Results of the present study showed that MSC have a significant antifibrotic effect as evidenced by the significant decrease in liver collagen gene expression as well as the decrease in hydroxyproline content in the CC14/MSC group (p < 0.001) compared to the CC14 group. The Y-chromosome gene (sry) was detected by RT-PCR in the CC14/MSC group, but was not detected in control group and other groups. The CD29 gene was expressed in MSC culture, and this confirmed the efficiency of isolation and propagation of MSC in culture. With regard to liver function, there was also a significant improvement and elevation of serum albumin in the CC14/MSC group compared to the CC14 group (p < 0.05). As regard to the liver enzyme ALT, there was a decrease of its level in the CC14/MSC group compared to the CC14 group. However, this was statistically nonsignificant (p > 0.05). In conclusion, MSC have a potential therapeutic effect against the fibrotic process through their effect in minimizing collagen deposition in addition to their capacity to differentiate into hepatocytes. (c) 2007 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.