The immune response to maedi-visna virus has been investigated, both in persistently infected sheep and in the acute phase of the primary immune response to infection with maedi-visna. Maedi-visna infected sheep develop detectable levels of anti-viral antibody by 4-6 weeks after experimental infection. These antibodies are directed against the envelope and core structural proteins of the virus and initially are of the IgM isotype, but later switch to IgG. These IgG anti-visna antibodies are restricted to the IgGj subclass. The functional significance of this isotypic restriction of the anti-visna antibody response was studied using in vitro assays of antibody activity against virus infected cells. It was found that visna specific antibodies were able to direct antibody-mediated complementdependent cytotoxicity, but not antibody dependent cell mediated cytotoxicity against virus infected cells. These observations are consistent with the known properties of ruminant immunoglobulin G subclasses. These persistently infected sheep were also shown to have developed a CD4+ T cell response to maedi-visna virus. The acute phase of the immune response to maedi-visna infection was studied in a lymphatic cannulation model. Infection with maedi-visna induced both virus neutralising antibodies and vims specific T cells, but these failed to prevent the establishment of a persistent viral infection. The generation of the data on immune responses to maedi-visna virus describe^ above was facilitated by the production of recombinant p25 gag protein and p25 specific polyclonal and monoclonal antibodies.