The results of serological testing for maedi-visna virus infection in indigenous and non-indigenous sheep in Northern Ireland, over a five year period (1980 to 1984) are presented. In tests carried out in 1980 and 1981 on pedigree breeds, 10 reactors were identified on seven farms and maedi-visna virus was isolated on three occasions from leucocyte cultures. None of the animals showed clinical signs of maedi-visna and all the reactors were subsequently destroyed. Nine of the reactors were imports from Scotland or the Republic of Ireland and the other was the progeny of an imported ewe. In subsequent tests (1982 to 1984) of exotic and indigenous flocks no further reactors were identified. A survey of goat herds for evidence of caprine arthritis-encephalitis virus infection was also negative.