The chinchilla's acute mortality response to mixed gamma-neutron radiations of the AFRRI-TRIGA reactor and 250-kVp x-rays was studied. Unilateral whole-body irradiations were accomplished at doses from 200 to 639 rads of mixed gamma-neutron radiations and from 357 to 1786 rads of x-rays. All radiations were delivered at approximately 20 rads/min and reported as midline doses. The ${\rm LD}_{50(30)}$ values calculated for the mixed gamma-neutron radiations and for the x-rays were 295 rads and 490 rads, respectively. With 30-day median lethality as the end point, the RBE of the mixed gamma-neutron radiations was 1.7. The wide lethal dose range obtained was attributed to a high degree of variation in age and weight of the chinchillas. At the supralethal doses of radiation associated with the gastrointestinal mode of radiation death in mice and rats (3 to 5 days mean survival times), the mean survival times for the chinchillas ranged between 6.3 and 9.6 days.