Lung adenomatosis of sheep (jaagsiekte) (PCS) originates from the terminal bronchiolar epithelial cells that also form type B alveolar epithelial cells. There is considerable evidence that this neoplasm is horizontally transmitted. Virus particles similar to RNA viruses have been found in PCS, while in tissue culture systems, herpes-like particles were encountered as well. Among lung diseases of sheep frequently associated and occasionally confused with PCS are included progressive pneumonia and maedi-visna disease. Pathologically these two diseases are diagnosed in sheep as progressive interstitial pneumonia and/or meningoencephalitis. Progressive pneumonia and maedi-visna disease are both caused by C type RNA viruses - PPV in the former and MV or VV in the latter - which serologically, structurally and functionally are similar if not identical. Molecular hybridization with radioactively labeled DNA complementary to the RNA of the maedi virus was used to probe for homologous RNA in the polysome fraction of pulmonary carcinomas (jaagsiekte) of Awassi sheep. No sequence homology was detected, which suggests that maedi (or visna) virus is not implicated in the etiology of pulmonary carcinoma of sheep.