The action of X Rays on the dried virus of myxoma of the rabbits has been studied. When the incidence of X-Rays reaches the concentration of 294.000 r to 378.000, a dosage which results in the destruction of all the pathogenic activity of the virus, not all the inoculated animals acquired the disease, which, when developing; has in several of these animals, a much slower than in the control animals. In accordance, with this symptomatology, the histopathological study of the material collected on the point of where the lesions are more marked in the animals suffering from a myxomatous infection of slow evolution, shows that the lesions are less extensive and less severe that the ones produced in the animals inoculated with normal virus. This is an indication that the X-rays induced an attenuation of the virulence of the virus of myxoma, but not a mutation. The animals inoculated successively with virus irradiated with more than 378.000 r, than inactivated, were after 30 days, inoculated with entirely virulent virus and acquired infectious myxomatosis with all its typical characteristics. This fact shows that the virus of myxoma inactivated by the X-Rays has not kept its antigenic properties and does not confer any protection against subsequent inoculations of virulent myxomatous virus.