A virus with growth and morphologic characteristics of progressive pneumonia (maedi-visna) virus was isolated from the lungs of sheep with typical clinical and postmortem changes of chronic progressive pneumonia. The virus grew slowly in cultures of embryonic ovine lung cells, causing syncytial formation and degeneration. Syncytia developed much slower and involved fewer cells than reported for other similar viruses isolated from sheep. As seen with the electron microscope, the virus reproduced by budding from cell surfaces. Two types of virions were seen-a large particle (120 to 140 nm) with an electron-lucent center and dense laminated outer rim, and a small particle (80 to 110 nm) with an electron-dense core surrounded by a single membrane. Viral structures and fragments similar to the large extracellular particles were seen in the cytoplasm of a few cells. These characteristics are reported for other viruses isolated from sheep with progressive pneumonia.