Titrations of anti-hepatitis C core (anti-HCc) immunoglobulin G (IgG) antibodies and its subclasses were studied in 90 patients with acute and chronic hepatitis C virus (HCV) infection, including 27 patients who underwent interferon (IFN) therapy. The positivity rates for each anti-HCc subclass were as follows: 95.2% for IgG1, 12.0% for IgG2, 69.9% for IgG3 and 19.3% for IgG4. The total anti-HCc IgG titre correlated well with the IgG1, titre, indicating that IgG1 was the main virus-specific IgG. Changes of IgG1 production mainly contributed to fluctuations of the anti-HCc IgG titre and corresponded well to positivity for HCV-RNA during and after IFN therapy. IgG3 was detected prior to IgG1 during the early phase of acute hepatitis in some cases and also appeared with relapse after IFN therapy. The serial assay of anti-HCc subclasses showed the patients' humoral immune response to HCV infection, and might be useful for evaluation of anti-viral immunity influenced by IFN therapy.