Oligonucleotides complementary to the sequences containing the initiator codon, AUG, of the core region of positive-stranded hepatitis C virus (HCV) were tested for their effects on viral translation in a cell-free protein synthesis system and on viral replication in a human T-lymphotropic virus type I infected cell line, MT-2C, which was cloned by the limited dilution method from MT-2 cells and showed more efficient HCV replication than an uncloned population of MT-2 cells. Treatment of HCV-infected MT-2C cells with the antisense oligonucleotide (10 microM) had a dramatic inhibitory effect on viral replication. This result suggests that the antisense oligonucleotide complementary to the sequence close to the initiation codon of the core region might be useful as an antiviral agent against HCV replication.