The structural polypeptides of the RNA slow viruses Visna, Maedi, Progressive Pneumonia Virus, and Zwoegerziekte have been determined by electrophoresis in polyacrylamide gels in a discontinuous SDS system. Identical patterns were obtained of 15 polypeptides of molecular weight ranging from 14,000 to 140,000 daltons. Most of the mass of protein is in three small molecular weight polypeptides; the major polypeptide species has a molecular weight of 25,000. A distinctive glycopeptide of molecular weight 140,000 is present in each virus; three additional glycopeptides were found, one of which is barely detected in radiolabeling experiments. There is one phosphorylated polypeptide. In a comparative study of RNA slow viruses and tumor viruses, 9 of 15 polypeptides of visna virus comigrated with 13 polypeptides of Rous sarcoma virus including the major 25,000 MW polypeptide of each virion. In addition, the isoelectric point of visna was 3.8 in conformity to the isoelectric point reported for Rous sarcoma virus. Distinctive small molecular weight polypeptides and a totally different pattern of glycosylation constitute the main differences of the polypeptides of these two groups of viruses. Extensive studies of the purification procedures for C-type viruses were carried out and criteria for virus purity delineated. The expectations for the relationship of the structural polypeptides to antigenic determinants are discussed.