The antigenic similarity between molecules of recombinant human interferon-alpha 1 (IFN-alpha 1) and recombinant human IFN-alpha 2 was demonstrated with neutralizing monoclonal antibody (mAb) 1-46. The common epitope for the mAb 1-46 was localized into amino-terminal region of IFN-alpha molecule around residues 30-35. Following pH 2 treatment, the biological activity of both IFN-alpha 1 and IFN-alpha 2 was retained but the antigenic relatedness between corresponding sequences 30-35 was diminished. The common structure on the IFN-alpha 1 molecule proved acid stable and the mAb 1-46 retained the ability to neutralize the pH 2 treated IFN-alpha 1. However, the neutralization of pH 2-treated IFN-alpha 2 by specific antibody was completely suppressed. These results complemented our earlier finding of the dramatic effect of acidic pH on the antigenic structure of region 132-137 of the IFN-alpha 2 molecule. We conclude that pH 2 may induce a conformational rearrangement of the IFN-alpha 2 molecule, resulting in an altered tertiary structure with deviating antigenic characteristics.