To solve the diagnostic problem of CSFV in laboratory about interference by the lapinized swine fever vaccine virus, this plan will establish a RT-PCR method to differentiate field isolates and lapinized vaccine strain of classical swine fever virus from tissue samples. This plan aims at the DNA-sequencing difference of Lapinized swine fever vaccine virus and field isolate CSFV by the characteristic of the 3'-untranslated region on the nucleic acid of the CSFV lapinized vaccine strain, which comprising a 12-13 bp nucleotide (poly T) insert in comparison with the field isolate CSFV. We will design the special primers at the both sides of insertion and use the RT-PCR or nested-PCR method to amplify this target gene and separate the PCR products by electrophoresis for direct analysis of the results.