Sustained hepatic inflammation induced by various causes can lead to liver fibrosis. Transcription factor NF-B is important in regulating inflammatory responses, especially in macrophages. We presently investigated whether an NF-B decoy, a synthetic oligodeoxynucleotide (ODN) imitating the NF-B binding site, inhibited the inflammatory response after CCl₄ intoxication to prevent CCl₄-induced hepatic injury and fibrosis. The NF-B decoy was introduced into livers by injecting the spleens of mice, using a hemagglutinating virus of Japan (HVJ)-liposome method. ODN was transferred mainly to macrophages in normal or fibrotic livers. Increases in serum transaminases and production of inflammatory cytokines after a single challenge with CCl₄ were inhibited by the NF-B decoy, which suppressed nuclear translocation of NF-B in liver macrophages. Liver fibrosis induced by CCl₄ administration for 8 weeks was suppressed by the NF-B decoy, accompanied by diminished mRNA expression for transforming growth factor (TGF)-1, procollagen type1 1, and -smooth muscle actin (SMA). In vitro, isolated liver macrophages showed increased DNA binding activity of NF-B and inflammatory cytokine production after hydrogen peroxide treatment; both increases were inhibited significantly by the NF-B decoy. In contrast, NF-B decoy transferred to isolated hepatic stellate cells (HSC) had no effect on their morphologic activation or -SMA expression, although the decoy accelerated tumor necrosis factor (TNF) -induced apoptosis in activated HSC. The effect of NF-B decoy suppressing fibrosis probably results mainly from anti-inflammatory effects on liver macrophages, with a possible minor contribution from its direct pro-apoptotic effect on activated HSC.