The polymerase chain reaction (PCR) was applied to detect bovine viral-diarrhoea mucosal-disease virus (BVDV). By the use of properly prepared primers, cytopathogenic NADL, Oregon C24V, Nose, T-20 and KS86-1(+) strains, and non-cytopathogenic New York-1, No. 12, and KS86-1(-) strains could be detected. The PCR system was applied to field isolates of the viruses. All the viruses were detected by the PCR. Four patterns of the PCR amplification were recognized, and it was possible to discriminate between some strains. These results corresponded with the serotype of BVDV, as determined by the serum-neutralizing test. The BVDV gene was detectable from the leucocytes of infected cattle using the PCR method. Moreover, it was possible to detect and discriminate BVDV strains using one PCR tube that included all primer pairs.