To test the multiplex PCR for diagnosis of porcine viruses, 76 clinical specimens were tested for PCV-2, CSFV, PPV and PRRSV using the multiplex PCR and confirmed by a routine PCR/RT-PCR using the same four sets of specific primers and DNA sequencing of PCR products (Table 2). The results were similar, except for one specimen that was positive for PRRSV by routine RT-PCR, but negative by multiplex PCR. The 42 specimens that were positive by multiplex PCR were also positive by conventional PCR assays and the 33/34 specimens that were negative by multiplex PCR were also negative by conventional PCR assays. Co-infection with two or three viruses was demonstrated in 26 specimens (34.2%); however, no specimens were co-infected with all four viruses (Table 3).