Three hepatitis C patients out of 15 who received recombinant interferon-alpha 2a (rIFN-alpha 2a) therapy developed high levels of neutralizing antibody coincident with clinical relapse. On analysis, antibodies in their sera were found to also produce some neutralization of lymphoblastoid IFN. Purified lymphoblastoid IFN was separated chromatographically into 10 fractions, each containing one or two IFN-alpha subtypes, and these were used individually to examine the subtype specificity of the neutralizing antibodies in the patients' sera. All three sera neutralized all the subtypes present in the 10 lymphoblastoid IFN fractions. These findings may explain why some patients who develop antibody-mediated resistance to treatment with rIFN-alpha 2 preparations are unable to respond again when treated instead with a human cell derived preparation containing many IFN-alpha subtypes.