The reservoir host of cowpox virus in Western Europe is not known, but epidemiological evidence from human and feline infections indicates that the virus is probably endemic in small wild rodents. Therefore, serum and tissue samples were collected from a variety of wild British mammals and some birds, and tested for evidence of Orthopoxvirus infection. Antibody reacting with cowpox virus was detected in 9/44 (20%) bank voles (Clethrionomys glareolus), 8/24 (33%) field voles (Microtus agrestis), 17/86 (20%) wood mice (Apodemus sylvaticus) and 1/44 house mice (Mus musculus), but in no other animal species tested. Although virus was not isolated from any animal, this serological survey, together with other evidence, suggests that bank and field voles and wood mice are the main reservoir hosts of cowpox virus in Great Britain.