We developed approaches using antisense RNA to inhibit hepatitis C virus (HCV) RNA translation and HCV core protein expression. An HCV genotype 1b cDNA comprising nt 1-1321 or a fusion construct consisting of HCV (nt 1-584) and luciferase cDNAs were inserted downstream of T7 and CMV promoter sequences and used to generate HCV RNA target molecules. Such constructs will produce HCV core or HCV coreluciferase fusion proteins in vitro or within transfected cells. Seven different antisense RNA constructs were designed to target the highly conserved 5' region of HCV RNA at nt positions 1-402. For in vitro experiments, synthesized HCV RNA target sequences and antisense RNAs were mixed at various molar ratios and subsequently translated in a rabbit reticulocyte lysate system. In cell culture studies, the HCV core-luciferase fusion cDNA was co-transfected with antisense RNA-producing constructs into human hepatocellular carcinoma (HCC) cells. Luciferase activity in cell lysates was measured to determine quantitatively antiviral effects within the cell. It was found that translation of HCV RNAs was efficiently inhibited by antisense RNA in vitro. The specificity of this inhibition was confirmed using control target RNA sequences or nonrelevant antisense RNA constructs. Co-transfection studies demonstrated that antisense RNA inhibited HCV core-luciferase fusion protein expression by 41-57% in HuH-7 HCC cells. These studies indicate that antisense RNA will find viral target RNA sequences in HuH-7 cells and inhibit HCV RNA translation. More important, these studies have defined critical viral RNA target sequences susceptible to antisense inhibitory effects within the cell.