Using glycerol gradient centrifugation, the molecular sizes of porcine (2′-5′)oligoadenylate synthetases (2–5A synthetases) were estimated. The 2–5A synthetase purified from pig spleen was about 150 kDa, while the enzyme extracted from nuclei of Newcastle disease virus-infected pig epithelial cells (SK-h) was about 20–40 kDa. The nuclear 2–5A synthetase was selectively adsorbed to Protein A-Sepharose beads conjugated with anti-spleen 2–5A synthetase antibody. Thus, the smaller 2–5A synthetase in nuclei of pig cells shares a protein structure with the larger enzyme from pig spleen.