Cell-free suspensions of papilloma virus (Shope) required for their inactivation in vitro amounts of Roentgen irradiation that are much greater than those needed to inactivate other infectious agents previously described. These amounts, millions of r units, are several thousand times greater than those required to eradicate permanently papillomas induced by the virus in domestic rabbits. Large doses of Roentgen radiation reduce the titer of papilloma virus, lengthen the period of time between inoculation and the appearance of papillomas, and decrease the size attained by the papillomas.

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