The sequence encoding the viral leader proteinase Npro was replaced by the murine ubiquitin gene in a full-length cDNA clone of the classical swine fever virus (CSFV) strain Alfort/187. The recombinant virus vA187-Ubi showed growth characteristics similar to those of the parent vA187-1 virus. At two occasions cells infected with vA187-Ubi exhibited a cytopathic effect and were found to contain a subgenomic viral RNA. This RNA lacked the same viral genes as the subgenomic RNA which has been found in all cytopathogenic CSFV strains analyzed so far, but it maintained the ubiquitin sequence.