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Cell cultures of Syrian hamster embryo were treated for 7 days with selected chemicals. Certain cultures were morphologically transformed by three different chemical preparations and yielded cell lines that subsequently produced malignant tumors in hamsters. Although the cell lines were negative for infectious virus before inoculation into animals, hamster-specific C-type RNA virus was isolated from tumors or from cell lines derived from the tumors. Since infectious C-type viruses are usually not demonstrable in hamster tissues of normal or tumor origin, we conclude that the chemical treatment and activation of the viruses are related events.