In the present paper the authors refered the experiments made with the lymphocytic chriomeningitis virus. We strain, after exposition to 4 tubes of 1 mg of radium. The virus suspension was put into the Carrel flask in a layer of 0,1 cm. The titrations of the irradiated virus suspension were made after 24 hours, 8, 20, 33, 78, 85, 120, 264, 292, 387 and 535 days. the virus was still active after 264 days, not after 292 days. The virus irradiated during 24 hours presented the same pathogenicity form mice than the control but after 8 days it was lesser and after 20 and 33 days it was enchanced (Graphic 10). After 78 days the pathogenic power was the same for the irradiated virus and the control.