Studies were made on the oncogenic properties of a line of polyoma virus transformed hamster cells. Passage of the cells in vivo resulted in an increase in oncogenicity. Thus the latent period for the appearance of a tumor was decreased, and as few as 5 to 10 cells were now sufficient to produce a tumor in a young hamster. Tumors were invariably restricted to the subcutaneous site of injection. In contrast, polyoma virus itself caused widespread production of tumors throughout the animal. Histological examination of tumor tissue revealed marked differences between tumors caused by virus or virus-transformed cells. It was concluded that little or no metastasis of transformed cells could have occurred, and that insignificant rescue of the polyoma virus genome must have been induced in transformed cell tumors.Powassan virus, a group B arbovirus, did not show any oncolytic activity toward tumors induced by the polyoma virus transformed cells.