The antigenic properties of human hybrid IFN-alpha8(60)/alpha1(92)/alpha8 were compared with those of human IFN-alpha1 and IFN-alpha2 using monoclonal antibodies (mAb). Hybrid IFN demonstrated a significantly closer antigenic relationship to the subtype alpha2 than to the subtype alpha1. In particular, high homology was observed between antigenic structures located in the C-terminal domains (93-166) of IFN-alpha8 and IFN-alpha2, whereas the corresponding N-terminal receptor-binding domains (30-53) showed distinct antigenic characteristics. The 100% homology between IFN-alpha8 and IFN-alpha2 in the region 114-131 (helix D) indicated the role of this region in formation of the common antigenic structure. In IFN-alpha8/1/8, this shared antigenic structure was important for antiviral activity and exhibited immunodominant properties, consistent with functional and antigenic properties of the corresponding structure in IFN-alpha2. Based on this antigenic homology, we suggest that IFN-alpha8 and IFN-alpha2 are evolutionarily more closely related to each other than to IFN-alpha1. This study will contribute to a better understanding of evolutionary events in the human IFN-alpha family.