A filterable oncogenic factor was isolated from ascitic tumors induced in X-irradiated hamsters by inoculation of BHK 21 clone 13 cells. This agent reproduces a tumor in hamsters 22 days after inoculation. Its oncogenic characteristics are preserved even after the ascitic tumor cells have been subcultured in vitro.

The occurrence of R or H virus-like particles, both in tumor cells and in cultures, suggests a probable participation of such structures in the oncogenetic mechanisms of the aforementioned neoplasm.

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