We studied hepatic stellate cell proliferation in vitro. Peripheral blood mononuclear cells (PBMC) from patients with chronic active hepatitis C (CAH) and liver cirrhosis (LC) were cultured for 24 h in the presence or absence of Escherichia coli lipopolysaccharides (LPS). Hepatic stellate cell proliferation induced by the culture supernatants was measured, and interleukin-1 (IL-1) and IL-6 levels in the culture supernatants were quantified. Culture supernatants of LPS-stimulated PBMC from LC patients induced rat hepatic stellate cell proliferation by almost 2.8-fold (stimulation index, 2.83 ± 1.41) compared with when the cells were cultured without addition of PBMC culture supernatants. Production of IL-1β was significantly higher in the culture supernatants of both CAH and LC patients than in those of ten healthy controls (P < 0.01 and P < 0.05, respectively). But there was no significant correlation between IL-1 production and the induction of hepatic stellate cell proliferation by the culture supernatants. Although there were no significant differences in IL-6 production by LPS-stimulated PBMC among healthy controls and CAH and LC patients, we observed a significant correlation between IL-6 production and the induction of hepatic stellate cell proliferation in the culture supernatants of LC patients. Rat hepatic stellate cells themselves produced IL-6, and treatment with IL-6 antisense oligodeoxynucleotides suppressed the cell proliferation, suggesting that IL-6 is an autocrine growth factor for hepatic stellate cells. The addition of human recombinant IL-6 (hrIL-6) augmented rat hepatic stellate cell proliferation, indicating that excessive IL-6 may further facilitate cell proliferation. These findings suggest that a cytokine cascade including IL-6 may participate in hepatic stellate cell proliferation in LC patients when they are exposed to endotoxin.