The maedi-progressive pneumonia-visna viruses are biochemically, antigenically, biophysically, and morphologically indistinguishable. The development of a plaque assay for these viruses has enabled us to demonstrate that the strain K796 visna virus and the strain M-88 maedi virus both contain at least two biologically different virus populations, large-plaque-forming (Lpf) virus and small plaque-forming (Spf) virus. Additional results indicate that visna virus, maedi virus, and progressive pneumonia virus are phenotypically distinguishable. Studies are being performed to determine if other visna, maedi, and progressive pneumonia strains contain biologically different virus populations, if clones of visna virus, maedi virus, and of progressive pneumonia virus can be distinguished from each other by their phenotypic properties, if the Lpf virus and Spf virus obtained from strain K796 visna virus contain visna virus-specific antigens, and if these visna virus variants replicate and persist in sheep. This last determination may suggest that one variant of visna virus is neurotropic and responsible for the slow neurological disease.