An ex vivo liver slice-culture method was used to develop a novel assay of pretherapy interferon-based suppression. To evaluate its clinical application, 45 consecutive patients with chronic hepatitis C were included. They underwent the ex vivo antiviral assay, followed by a 24-week course of therapy with peginterferon and ribavirin. A stepwise logistic regression model was used to estimate the relationship between sustained virological response and the presence of various clinicopathological parameters. The results indicated that the presence of an interferon-based suppression effect in the ex vivo assay (odds ratio [OR], 12.454 [95% confidence interval {CI}, 1.115-139.090]; P=.0405), clinically diagnosed liver cirrhosis (OR, 0.081 [95% CI, 0.011-0.584]; P=.0126), and the portal-inflammation score (OR, 4.220 [95% CI, 1.264-14.085]; P=.0192) were independent determinants of sustained virological response. In conclusion, this new assay serves as an independent predictor of sustained virological response in interferon-based antiviral therapy.