A template-bound RNA polymerase was isolated from Nicotiana clevelandii plants infected with red clover necrotic mosaic dianthovirus (RCNMV) by differential centrifugation, solubilization with dodecyl beta-D-maltopyranoside, and chromatography on columns of Sephacryl S-400 and Q-Sepharose. Analysis of the purified polymerase by SDS-polyacrylamide gel electrophoresis, followed by silver staining or immunoblotting, showed that it contained virus-encoded proteins of molecular masses 27 kDa and 88 kDa together with several minor proteins possibly of host origin. After removal of endogenous RNA with micrococcal nuclease, the polymerase became template-dependent. It was also template-specific, being able to utilize as templates RNA of two strains of RCNMV, but not RNAs of three viruses in different taxonomic groups, namely cucumber mosaic cucumovirus, tomato bushy stunt tombusvirus and tomato mosaic tobamovirus. The products of RNA polymerase reactions were double-stranded RNAs corresponding to RCNMV RNAs 1 and 2. The ability of the template-dependent RNA polymerase to synthesize RNA was completely inhibited by antibodies to a peptide containing the GDD motif, whereas the activity of the template-bound enzyme was unaffected by these antibodies.