A cloned line of Syrian hamster kidney cells (C13) was infected with polyoma virus and clone C13-TC6 was prepared from one of the resulting transformed cells. A comparison between the transformed clone and uninfected C13 cells revealed that the following characteristics were acquired by the transformed cells: (a) a higher plating efficiency; (b) a higher production of nonvolatile acid in culture, possibly indicative of enhanced glycolytic activity; (c) an ability to initiate tumors in hamsters when 106 or more cells were inoculated subcutaneously; and (d) a tendency to accumulate chromosome abnormalities and tetraploid cells, though these abnormalities were not immediately associated with transformation. Both the C13-TC6 line and the uninfected control line C13 produced tumors when injected into the cheek pouch of the hamster.—J Nat Cancer Inst 30: 795–815, 1963.