Only limited data are available on the development of neutralizing antibodies (NAB) in patients with chronic hepatitis C (CHC) treated with pegylated interferon- (PEG-IFN-). The aim of this study was to evaluate the immunogenicity of PEG-IFN- when administered to CHC patients who had or had not previously received standard IFN- therapy. In addition, the specificities of NAB, together with the ability of leucocyte (LE) -IFN- to re-establish therapeutic responsiveness in NAB-positive patients, were evaluated. NAB were assessed using a quantitative, standardized, virus-induced cytopathic effect assay. The seroconversion rate to PEG-IFN- was higher in patients who had received previous standard IFN- treatment than in those treated exclusively with PEG-IFN-. Also, NAB produced during PEG-IFN- therapy were unable to neutralize LE-IFN- entirely, even though they can neutralize several IFN- subtypes. In addition, the results indicate that a change to LE-IFN- therapy can be associated with restoration of clinical responses in NAB-positive patients who had become resistant after showing an initial response to PEG-IFN- treatment. This study emphasizes the importance of evaluating NAB development in CHC patients who become resistant to PEG-IFN- treatment, and suggests management alternatives for patients who develop NAB