In mongrel dogs, a combination of 100 mg of aminoethylisothiuronium'' Br [center dot] BHr per kilogram orally and 3 mg of p-aminopropiophenone per kilogram intraperitoneally, immediately prior to a total-body X-ray insult of 500 r, provided a high degree of radiation protection. Either drug alone did not improve survival rate over that of control animals. Evidence of a moderate degree of protection by the same combination against a total-body dose of 800 r was also apparent. The protective effects of the 2 agents against otherwise lethal radiation appear to be additive.