Knowledge of the dynamics of hepatitis C virus (HCV) in vivo is important for elucidation of its pathogenesis and the establishment of therapeutic guidelines. The aim of this study was to obtain kinetic information about virus load following interferon-alpha (IFN-alpha) administration. Serial serum HCV core protein and HCV RNA levels were measured. IFN-alpha exponentially reduced serum HCV levels. The mean (+/-SD) viral half-life was 7.0 +/- 2.6 h in HCV core protein assay and 7.2 +/- 3.1 h in HCV RNA assay on the first day of therapy. This initial rapid decrease was followed by a slower decrease in serum HCV levels thereafter. Thus, the biphasic reduction in virus load during IFN-alpha therapy was demonstrated.