With <tex-math>$1\text{-}{\rm MVp}$</tex-math> X-rays, the acute <tex-math>${\rm LD}_{50(30)}$</tex-math> (midline air exposure) of 8- to 9-month-old Duroc gilts was determined to be 399 R (95% confidence interval: 371 to 424 R). The pattern of recovery from the effects of 240 or 265 R of X-rays was studied by means of the split-dose technique, which consists in conditioning the animals with a sublethal exposure and redetermining the LD50 at various times thereafter. By this criterion, the swine had recovered from 51% of the initial injury by 3 days. By 7 days, they appeared to have recovered from about 65% of the initial injury, but the data indicate a considerable heterogeneity of response at this time. By 20 days, the entire population appeared to be radioresistant; the redetermined LD50 was 164% of the LD50 of unconditioned animals. Radioresistance was also evident at 61 and 107 days: at 61 days, 734 R killed 8 out of 15 animals conditioned with 265 R, whereas the same exposure killed 15 of 15 unconditioned animals; at 107 days, none of 9 conditioned animals died after 399 R. The pattern of systemic recovery and the period of radioresistance are not clearly reflected in the pattern of hematological changes that follow a conditioning exposure.

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