The effect of IFN-alpha on the growth of 5 hepatoma cell lines and a normal liver-derived cell line were examined. IFN dose-dependently inhibited the growth of cell lines except for HLE and PLC/PRF/5 in which the inhibition only occurred at a high concentration over 10,000 IU/ml. IFN-alpha induced the G1 arrest of these cells according to upregulation of p21WAF-1 expression, which is induced in PLC/PRF/5 and HLE only at a high IFN concentration. The receptor for IFN-alpha was well expressed in all the cell lines. DNA rearrangement of IRF-1 was detected in HLE and PLC/PRF/5 by Southern blotting, while IRF-2 gene was preserved. IFN-induced gene expressions were compared between HCC-T, HCC-M and PLC/PRF/5. RT-PCR demonstrated that the full-length IRF-1 and -2 mRNA was transcribed in all cell lines, but the mRNA amount of former gene in PLC/PRF/5 was less than that in HCC-T and HCC-M, about 1/10 by competitive RT-PCR. The sequence analysis of IRF-1 cDNA was performed and the full-length mRNA transcription was reconfirmed in PLC/PRF/5, but no significant point mutation was detected. These results suggest that IFN-alpha inhibits hepatoma growth by increasing p21WAF-1 expression only when the IRF-1 gene is preserved normally.