Polypeptides having 2',5'-oligoadenylate phosphodiesterase activity, antibodies against the polypeptides, polynucleotides coding for the polypeptides, recombinant plasmid DNAs into which the polynucleotides have been inserted, host cells transformed by the recombinant plasmid DNAs, processes for screening for 2',5'-oligoadenylate phosphodiesterase activity inhibitor substances, and processes for screening for 2',5'-oligoadenylate phosphodiesterase expression regulation inhibitor substances. The polypeptides, polynucleotides and host cells are useful for searching for therapeutic agents for viral infections and tumors.