Abstract: The conventional and clean KM，BALB/c and C57BL/6J mice were detected for LCM（ lymphocytic choriomeningitis virus）, EHFV（ epidemic hemorrhagic fever virus），Mouse pox, MPV（ mouse hepatitis virus）, Sendai virus infetion by using IFA，ELISA,ABC（ avidin-biotin-peroxidse complex）, and electron microscope before and 15,30，60 days after undergoing animal experiments.The results revealed that 60 days after the animal experiments, ectromelia virus was found in the liver，mesenteric lym-phaden，Payer’s patches, and spleen of seronegative（ELISA，IFA ） KM and BALB/c mice，while in the other groups no ectromelia virus infection were found.The findings suggest that the mice with latent infection of ectromelia virus,despite of sero-negative,would give rise to an outbreak when they were under a long stress of animal experiment ,and the viral antigen detection of Mousepox should be used in routine microbio-logical monitoring.