Interferon-alpha (IFNalpha) is not only an immunoregulatory factor, but is also an analgesic molecule. We ever reported that there exist distinct domains in IFNalpha molecule that mediate immune and analgesic effects respectively and inferred that the analgesic domain locates around the 122nd Tyr residue of IFNalpha molecule in the tertiary structure. After the 36th Phe residue, which was located closely to the 122nd Tyr residue in the tertiary structure, was mutated to Ser using site-directed mutagenesis, the analgesic activity of this mutant lost completely, but the antiviral activity of IFNalpha still maintained 40.5% of wild type IFNalpha. The results suggest that the 36th Phe residue is one of the constituent for the analgesic domain of IFNalpha and inferred that the analgesic domain of IFNalpha consists of the 122nd Tyr and the residues around the 122nd in the tertiary structure, which include the 36th Phe.