Liver fibrosis is characterized by an increased deposition of extracellular matrix proteins, including collagen type I, by activated hepatic stellate cells (HSCs). Previous studies have shown that this increase is mediated primarily by a post-transcriptional mechanism. In particular, the RNA-binding protein alphaCP binds to the alpha1(I) collagen 3'-untranslated region (UTR) and stabilizes this RNA in activated, but not quiescent, HSCs. This study examines the role of alphaCP in the decay of transcripts containing the collagen 3'-UTR in extracts obtained from NIH fibroblasts and quiescent and activated HSCs. Using an in vitro decay system, alphaCP binding activity was competed out with the addition of wild type oligonucleotides, but not with mutant oligonucleotides. Competition of alphaCP binding activity increased the rate of decay of wild type transcripts containing the alphaCP 3'-UTR binding site, but not of transcripts containing a mutated binding site. Quiescent HSC extracts contain no alphaCP binding activity and have no difference in the rate of decay of transcripts with wild type and mutant binding sites for alphaCP. The addition of recombinant alphaCP was sufficient to increase the half-life of the wild type transcript, whereas that of the mutant transcript was minimally changed. In vitro decay assays performed with activated HSC extracts that contain alphaCP binding activity demonstrate a markedly reduced decay rate of wild type compared with mutant transcripts. In vivo small interfering RNA experiments targeting alphaCP showed a reduction of the binding activity of alphaCP and a concomitant reduction in intracellular levels of alpha1(I) collagen messenger RNA. In conclusion, this study demonstrates the direct role of alphaCP in the stabilization of alpha1(I) collagen messenger RNA by blocking RNA degradation in activated HSCs.