The Viral RNA extracted from cell infected with MHV1,MHV3,A59, JHM strains was transcribed into cDNA by using AMV reserse transcriptase, then t he cDNA was amplified by PCR. The MHV-specific fragments were obtained from all four strains with primer cpc1、2 and cpc3、4, but no band was oberserved in IBV standard and wild strains; A 241bp fragment was obtained from all four strains a nd IBV with primer cpc5、6. Virus RNA solutions were serially diluted and subjec ted to the RT-PCR analysis. The 10pg of viral RNA was detected.