Hepatitis B, C, and D viruses can infect liver cells and in some individuals establish a chronic phase of infection. Presently, relatively little information is available on the antiviral mechanisms in liver cells. Because no good in vitro model infection systems for hepatitis viruses are available, we have used influenza A, Sendai, and vesicular stomatitis (VSV) viruses to characterize interferon (IFN) responses and IFN-induced antiviral mechanisms in human hepatoma cell lines. HepG2 or HuH7 cells did not show any detectable IFN-α/β production in response to influenza A or Sendai virus infections. Treatment of cells with IFN-α resulted in upregulation of IFN-α-inducible Mx, 2′,5′-oligoadenylate synthetase (OAS) and HLA class I gene expression but only with exceptionally high levels of IFN-α (≥100 IU/ml). Accordingly, high pretreatment levels of IFN-α, 1000 IU/ml for influenza A and VSV and 100 IU/ml for Sendai virus, were required before any detectable antiviral activity against these viruses was seen. IFN-γ had some antiviral effect against influenza A virus but appeared to be ineffective against VSV and Sendai virus. IFN-γ upregulated HLA class I protein expression, whereas Mx or OAS expression levels were not increased. There was a modest upregulation of HLA class I expression during Sendai virus infection, whereas influenza A virus infection resulted, after an initial weak upregulation, in a clear decrease in HLA class I expression at late times of infection. The results suggest that hepatoma cells may have intrinsically poor ability to produce and respond to type I IFNs, which may contribute to their inability to efficiently resist viral infections.