The authors refered in the present paper the experiments made with poliomyelitis virus, type 2, strain MEF1, after exposition to X Rays in the Carrel flask in a fluid layer of 0,1 ml of the vírus suspension. The titrations of the irradiated virus were made after the exposition of the virus suspension from 307 r to 3.408.500 r. The authors observed irregularity in the behaviour of the irradiated virus in comparison with the control. The virus was less active after exposition from 1.021.000 r to 3.386.000 r than the control but still pathogenic for mice. After exposition to 3.408.500 r the virus was inactivated.