Cellular responses to DNA of SV40 virus were more accentuated by inoculation of cell free filtrates of BSC-1 cells infected with DNA than of DNA alone. Essential histologic findings induced by DNA developing viral agents were similar to the cell responses to DNA. 39 of 128 hamsters (about 30 per cent) developed tumors within about 200 days post-inoculation of these DNA developing viral particles. Sarcomas were common and they were confined to the subcutaneous area inoculated the agents. From the experiment the findings of oncogenic capacity of DNA developing viral particles has attracted a considerable attention for the approach to resolve the course of cancer from the view of the infectious DNA.