A combined preparation of liver extract and flavin adenin dinucleotide (FAD) (Adelavin^®) has been widely used in patients with chronic liver diseases in Japan. One milliliter of this agent contains 15 μl of phenol-soluble phase of liver nucleic acid fraction and 10 mg of FAD. To examine the advantages of using this preparation in the elimination of hepatitis C virus (HCV) from patients with chronic hepatitis (CH)-C receiving interferon (IFN), 2 ml of this preparation was intravenously (n=9) or intramuscularly (n=8) administered daily for 5 days before 6 million units of IFN-α was intramuscularly injected once. Before and 48 hours after the injection of IFN, serum ALT, 2'5'-oligoadenylate synthetase (2'5-AS) activity, and HCV RNA levels were measured. The daily administration of this preparation alone for 5 days did not significantly change serum ALT, 2'S-AS activities, and HCV RNA levels. The 2'5-AS activities were significantly increased by IFN after the intravenous injection of this preparation (p<0.01), while an injection of IFN alone of this dose did not change its activities (n=10). HCV RNA levels were significantly decreased by IFN only after the administration of the preparation (intramuscular, p<0.01; intravenous, p<0.01). The effect of intravenous injection of this preparation was also elicited in patients with HCV genotype H and with HCV more than 10⁵ copies/ml. These results suggest that this preparation may enhance the 2'5-AS production by IFN as a result of the increase in mitochondrial adenosin triphosphate production and may be a potent agent to enhance the anti-viral efficacy of IFN in patients with CH-C.