Chronic intermittent injection of carbon tetrachloride (CCl 4 ) for more than 10 weeks induced liver fibrosis in mice, as evidenced by positive Azan staining and increased intrahepatic collagen content. Preceding the onset of liver fibrosis, interleukin-6 (IL-6) gene expression was enhanced in liver and immunoreactive IL-6 was detected in infiltrating inflammatory cells. To delineate the role of IL-6 in this process, we treated IL-6-deficient mice with CCl 4 in a similar manner for 12 weeks, after which fibrotic changes were less evident and serum albumin levels were lower in IL-6-deficient than wild-type mice. Moreover, CCl 4 -induced expression of transforming growth factor ␤ 1 and hepatocyte growth factor genes in liver was significantly reduced in IL-6-deficient mice. Thus, IL-6 may be vitally involved in fibrotic changes and maintenance of serum albumin levels, partly by modulating intrahepatic expression of these cytokines. J. Leukoc. Biol. 66: 601-608; 1999.