BNL Swiss Albino mice were exposed (five in tandem) in a 2.5-cm I.D. Lucite tube to a parallel beam of 2.2-BeV protons. The LD50 was 1.81 ±$0.03\times 10^{10}\ {\rm p}/{\rm cm}^{2}$, or 641 rads. The corresponding LD50 for 250-kVp x-rays was 557 rads, yielding an RBE of 0.87. No difference in time pattern of death was observed between the x-irradiated and proton-irradiated animals. It is concluded that, with the exposure geometry used in these experiments, ionization by primary and high-energy secondary protons was the major dose constituent. A comparison is made with other experiments on the lethal effects of protons in which different geometries were employed. There is evidence that, with exposure in material of larger diameter in which there is a larger contribution to dose from lateral scatter, high-LET components of the beam may play a more dominant role. It was also observed in these experiments that the presence of Pseudomonas aeruginosa may result in a lower LD50 and "early death," following either x-irradiation or proton radiation. This may have accounted for some of the "early deaths" following proton irradiation reported earlier.

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