It has been more than twenty years since the isolation of polyoma virus and SV40, and the reports that they could produce tumors in animals and transformation of cells in culture. What was startling was that these biologic properties are associated with viruses that contain genetic in- formation that is able to code for only five or six proteins. Since that time, investigations with these viruses have been in four principal areas. One major area of study has been on cells transformed by viruses that show altered growth properties and specify new viral and cellular proteins. Transformation...