It is known that IFN therapy can induce the development of anti-IFN antibodies. In order to evaluate the biological and clinical significance of both neutralizing (NA) and non-neutralizing (binding) antibodies, 123 patients with chronic hepatitis C treated with recombinant IFN-alpha were examined. Among them, 15 were positive for NA and 24 for binding antibodies. The kinetics of NA appearance show that, in general, they develop early during the first 3 months of treatment. Moreover, NA seem to be clinically relevant, since they may be responsible for non-responsiveness to treatment in 53% of patients who develop them. The evaluation of the clinical significance of binding antibodies is more difficult. They appear significantly earlier in non-responders than in responders, but no differences were observed in the overall percentage of seroconversion between responders and non-responders. Thus, it is not possible at the moment to establish their possible role in inducing non-responsiveness.