This proposal pertains to studies on two aspects of neoplastic transformation of cells in tissue culture by polyoma virus. One set of experiments deals with the understanding of the steps leading to the integration of the viral DNA into the host genome and the role of large T-antigen in this progess. Specifically, we will ask whether viral genomes are converted to high molecular weight concatamers after infection, whether recombination plays a role in the conversion, and, if so, whether the integrated viral molecules are selectively of recombinant nature and whether recombination is restricted to one area of the viral genome. Whereas most studies will be carried out with Fischer rat cells, rat NRK and hamster BHK will be used to compare the role of various hosts in some aspects of the integration process. The other set of experiments consists of comparative molecular studies on two groups of cells transformed by ts-a mutants, those with temperature sensitive and those with temperature insensitive transformed phenotype. The experiments will test the following points: 1) whether the temperature sensitive phenotype and insensitive phenotypes can be transferred to recipient cells by DNA transfections, 2) how a ts phenotype can evolve to a non-ts one, and 3) whether the expression of viral T-antigens is cell cycle dependent in temperature sensitive clones.