Classical swine fever virus (CSFV) E-rns is an envelope glycoprotein possessing RNase activity. The E-rns-based enzyme-linked immunosorbent assay (ELISA) has been considered a discriminating diagnostic test for differentiating infected from vaccinated animals. The purpose of this study was to produce a specific monoclonal antibody (MAb) to E-rns for further developing an indirect sandwich ELISA. The MAb CW813 was shown to specifically recognize both the monomer and dimer forms of Pichia pastoris yeast-expressed E-rns (yE(rns)). The antigenic site recognized by MAb CW813 was mapped to the region of amino acid residues 101-160 of E-rns where it was neither a neutralizing epitope nor essential to RNase activity. Furthermore, MAb CW813 was utilized as a capture antibody to develop a yE(rns)-based indirect sandwich ELISA for detecting swine antibody to E-rns. The assay demonstrated a high sensitivity and specificity that may provide an alternative method for developing a diagnostic kit with easy manipulation and low cost.