Precipitating antibody was demonstrated by the Ouchterlony technique in sera of sheep, intrapulmonary or intracerebrally infected with maedi-visna virus. Debris from virus infected cultures of sheep choroid plexus cells served as an antigen. The precipitating activity of the serum was located in the IgG-fraction. The course of precipitating antibody was investigated in 16 experimentally infected animals and 11 controls divided over three experiments, which were terminated after 14 to 16 months, 4 years and 5 1/2 years, respectively. Precipitating activity could be detected within 2 to 8 weeks after exposure and persisted for the entire duration of the experiments. In the experiments lasting 4 and 5 1/2 years, four animals were encountered in which a positive precipitin test could not be confirmed by typical macroscopic or microscopic lesions at autopsy, despite the isolation of virus from the blood at irregular intervals during the experiment and the isolation of virus from the spleen of one of these sheep at the time of autopsy.