Maedi virus has been isolated from sheep and serological tests, cytopathological data and biochemical examinations confirmed that the isolated virus was the maedi one. Viral particles were visible under an electron microscope in negatively stained samples prepared from the supernatant of infected cell cultures. The size of the particles corresponded to an average size of the maedi virus. The findings confirmed previous clinical, epizootic and serological data with regard to the existence of maedi disease in Poland.