Thirty-five patients with chronic viral hepatitis (B, D or non A, non B), treated with alpha interferon were evaluated by a highly sensitive enzyme immunoassay for the presence of alpha interferon neutralizing antibodies before and after three months of treatment. Although 4 subjects (11.4%) became immunized against alpha interferon, treatment with the drug was successful in inhibiting viral replication in 3 of them. We conclude that the appearance of anti alpha interferon antibodies does not affect the effectiveness of the drug, and is therefore not worth monitoring.