The present invention aims at providing new pestivirus mutants for use in vaccines. The present inventors worked with pestivirus mutants containing mutations in the coding region for the Npro protein of the virus. The mutants according to the present invention are characterized in that the mutant is based on a cp strain of the virus wherein part of the gene sequence encoding the Npro region is deleted, wherein said deleted part does not encompass the coding sequence for the N-terminal twelve amino acids of the Npro protein. Preferably the pestivirus is the Bovine Viral Diarrhea Virus (BVDV). Especially for BVDV it was found that a mutant according to the invention is a safe and efficient vaccine candidate.
