OUR GROUP. BACKGROUND/AIM: Heparin or heparin-derived molecules have been demonstrated to have a renoprotective activity in a number of experimental nephropathies and anti-fibrotic effect on pulmonary fibrosis induced by Bleomycin. However, the effectiveness of low anticoagulant activity heparin (LAAH) in the treatment of liver fibrosis has not been well defined. We are here demonstrating by both biochemical and morphological methods that long term LAAH administering can considerably decrease the hepatic fibrosis in rats elicited by carbon tetrachloride (CCl(4)) or injection of porcine serum. And its inhibition of fibrosis may be associated with the intervention of ERK signal transduction pathway and down-regulation of AP-1 activity in hepatic stellate cells. METHODS: LAAH were given to rats that subjected to CCl(4) or porcine serum injection induced liver fibrosis regimens up to 10 weeks. Immunohistochemical stainings for collagen types I and III plus image analysis as well as measurement of hydroxyproline content in the livers were carried out to evaluate the fibrosis of livers in rats. The activated HSCs (strong positive immunohistochemical staining for alpha-smooth actin) in the livers were counted under microscopy. The activities of extracellular signal-regulated kinase (ERK) in HSCs in vitro were estimated by the Western blot. Activator protein-1 (AP-1) activity in nuclear proteins of HSCs was measured by the electrophoresis motility shift assay (EMSA). RESULTS: The immunohistochemical staining plus image analysis showed that LAAH administration could reduce the contents of collagen types I and III to 16.48% and 27.94% of those in saline administration control in livers of rats treated with CCl(4) or 30.98% and 35.43% of those in saline administration control in livers of rats subjected to injection of porcine serum. Meanwhile, LAAH could decrease the content of hydroxyproline in the livers of rats treated with CCl(4) or injection of porcine serum by 32.83% or 32.41%, respectively. Long-term administration of LAAH can remarkably reduce the alpha-SMA positive cells only in the fibrotic livers induced by CCl(4). In vitro studies of HSCs showed that LAAH could inhibit the PDGF-BB augmented both expression and phosphorylation of ERK proteins and AP-1 activity in a dose-dependent manner. CONCLUSIONS: Long-term administering LAAH can suppress the hepatic fibrosis either induced by CCl(4) or injection of porcine serum. The mechanism of its anti-fibrotic effect may partly be related to the inference of ERK signal transduction pathway and AP-1 activity in activated hepatic stellate cells.