The${\rm LD}_{50(30)}$ (midline air exposure) of adult female burros exposed to 1 Mvp x-rays at a rate of 7.5 R/minute was determined to be 369 R; the${\rm LD}_{50(60)}$ was determined to be 344 R. The split-dose technique, which consists of conditioning animals with a sublethal exposure and redetermining the LD50 at various times thereafter, was used to determine the recovery pattern after an exposure to 250 R. By this method the burro appeared to have recovered from 48% of the conditioning injury at 60 days and from 85% at 90 days. A bimodal death pattern, with peaks at 2.4 and 24.2 days, was seen in animals given single spaced exposures. This pattern was not found in the animals that survived the conditioning exposure and were later subjected to spaced exposures. Comparison of the 1 Mvp x-ray${\rm LD}_{50(30)}$ value with earlier reported LD50 values established an apparent exposure rate-effectiveness relationship for the burro. The pattern of recovery at 60 and 90 days was found to closely parallel the return of white blood cells to normal values. The slow recovery has been suggested to be associated with the small numbers of circulating lymphocytes that were present after the conditioning exposure.

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