In the human fibroblasts culture, transformed by the material from malignant paraganglioma culture, there was found the RNA- and DNA containing virus synthesis. The RNA-containing virus with a buoyant density of 1.16--1.18 g/ml shows no hemagglutinating activity, it fails to infect newborn Syrian hamsters, and produces no transformation-destruction changes in mammalian tissue culture. The DNA-containing virus with a buoyant density of 1.24--1.28 g/ml would agglutinate the guinea-pig and group 0 human erythrocytes, produces lesions in newborn Syrian hamsters, and cytodestructive changes in the culture of rat and mice embryonal cells and in cells of a transplantable line Vero. In terms of its properties it may be referred to human viruses of the Papova group. A combined action of DNA- and RNA-containing viruses produces a transformation effect on embryonal hamster cells and the cells of a transplantable line Vero.