The occurrence of antibodies against recombinant human interferon-α2a (IFN-α2a) in patients with acute viral hepatitis (AVH) was examined by ELISA. Naturally occurring IgG anti-IFN-α2a were found in 50% of patients with type A, 50% of those with type B and in 8.3% of those with non-A, non-B AVH. The corresponding frequencies of IgM antibodies were 80%, 30% and 33.3%, respectively. IgM anti-IFN-α2a were found more frequently in patients with AVH type A than in normal control subjects (P < 0.01). Anti-IFN-α2a were detectable at the highest frequency 3 weeks after acute onset and then became negative. An absorption experiment revealed that IgM anti-IFN-α2a did not cross-react with recombinant human IFN-α2b. Immunoblotting analysis confirmed the binding of antibodies to IFN-α2a. Sera positive for IgG and/or IgM anti-IFN-α2a were unable to neutralize IFN-α2a. The appearance of anti-IFN-α2a was not correlated with disease severity. There was no evidence to suggest that anti-IFN-α2a impaired the elimination of hepatitis virus. This is the first study to demonstrate the occurrence of anti-IFN-α2a in patients with AVH. Detection of anti-IFN-α2a may be useful for clarifying any underlying immune events in various diseases.