Two regions of amino acids homologous to the ribonuclease catalysis domain of the fungal RNases T2 of Aspergillus oryzae and Rh of Rhizopus niveus and the plant S-glycoproteins of Nicotiana alata are perfectly conserved in the amino acid sequence of the envelope glycoprotein E2 of classical swine fever virus (CSFV). To analyze the functional significance of these conserved sequences, the gene encoding E2 was inserted into the p10 locus of baculovirus and expressed in insect cells. Recombinant virus BacCE2 generated a protein which was similar in size (42 to 46 kDa) to wild-type E2 synthesized in swine kidney cells infected with CSFV. Recombinant E2 was purified by immunoaffinity chromatography from the lysate of cells infected with BacCE2 and assayed for RNase activity. RNase activity coeluted with the E2 fraction, indicating that ribonuclease activity is an inherent property of E2. The ribonuclease-specific activity of the protein fraction containing pure E2 was comparable to that of the N. alata S-glycoproteins.