We investigated the effect of human umbilical mesenchymal stem cells (HUMSCs) from Wharton's jelly on carbon tetrachloride (CCl₄)–induced liver fibrosis in rats. Rats were treated with CCl₄ for 4 weeks, and this was followed by a direct injection of HUMSCs into their livers. After 4 more weeks of CCl₄ treatment (8 weeks in all), rats with HUMSC transplants [CCl₄ (8W)+HUMSC liver] exhibited a significant reduction in liver fibrosis, as evidenced by Sirius red staining and a collagen content assay, in comparison with rats treated with CCl₄ for 8 weeks without HUMSC transplants [CCl₄ (8W)]. Moreover, rats in the CCl₄ (8W)+HUMSC (liver) group had significantly lower levels of serum glutamic oxaloacetic transaminase, glutamic pyruvate transaminase, α‐smooth muscle actin, and transforming growth factor‐β1 in the liver, whereas the expression of hepatic mesenchymal epithelial transition factor–phosphorylated type (Met‐P) and hepatocyte growth factor was up‐regulated, in comparison with the CCl₄ (8W) group. Notably, engrafted HUMSCs scattered mostly in the hepatic connective tissue but did not differentiate into hepatocytes expressing human albumin or α‐fetoprotein. Instead, these engrafted, undifferentiated HUMSCs secreted a variety of bioactive cytokines that may restore liver function and promote regeneration. Human cytokine assay revealed that the amounts of human cutaneous T cell–attracting chemokine, leukemia inhibitory factor, and prolactin were substantially greater in the livers of the CCl₄ (8W)+HUMSC (liver) group, with considerably reduced hepatic inflammation manifested by a micro positron emission tomography scan. Our findings suggest that xenogeneic transplantation of HUMSCs is a novel approach for treating liver fibrosis and may be a promising therapeutic intervention in the future. Liver Transpl 15:484–495, 2009. © 2009 AASLD.