Sporozoite and trophozoite suspensions of Plamodium gallinaceum were exposed to graded doses of X-ray. In-fectivity of the irradiated suspensions was tested by inoculation into young chicks (Gallus domesticus). Trophozoites were inactivated by 20,000 roentgens but not by 10,000; sporozoites, by 8,000 r. but not by 6,000. Trophozoite suspensions exposed to 2,000 to 8,000 r. produced infectiona characterized by lengthened prepatent periods and lengthened survival times. Therapeutic irradiation of infected chicks had no significant effect on the outcome of the infection. A sample of human blood parasitized by P. malarie and exposed to 5,000 r. failed to produce an infection in a susceptible patient.