Hr-t mutants of polyoma virus are used as biochemical tools in a three-part investigation into the mechanism of cell transformation by this virus. The first part deals with genetic manipulations on the virus itself. Hr-t mutants appear to be defective in both middle-sized (56K) and small (22K) T antigens. To better understand the role of each of these proteins in the hr-t mutant phenotype, we shall attempt to isolate and study mutants with a normal 22K protein but abnormal 56K protein. The second part is an investigation of the possible roles of the various polyoma T antigens as protein kinases, cyclic-AMP binding proteins, or substrates for cellular protein kinases. The third part deals with the role of the hr-t gene in inducing chromatin modifications of both viral and cellular chromatin.