A virus was isolated from peripheral blood leukocytes of a cow which was kept in an isolated pen after it was injected with recombinant bovine interferon-gamma. The virus was identified as a member of genus Parapoxvirus in the family Poxviridae on the basis of electron microscopic observations and serological tests. Parapoxvirus has seldom been isolated other than from papular lesions, the characteristic sign of parapoxvirus infection. This is the first report of parapoxvirus isolation from the peripheral blood of a cow without any clinical signs. These results show that parapoxviruses are capable of causing persistent infection in cattle without clinical signs and can be activated by stress factors that induce modification of immune reactions. Relationships between the isolated virus and other parapoxviruses isolated previously from cattle in Japan were investigated and discussed.