Non-absorbable antibiotics, neomycin sulfate or polymyxin B, prevent death from an otherwise lethal dose of whole body radiation by suppressing the activity of the Gram-negative bacterial flora of the intestinal tract.

The protective effect of such suppression has been evaluated over a range of radiation exposure from 325 to 675 r. Coliform-free animals uniformly survive exposure to 550 r, a dose which is regularly lethal for coliform bearing animals.

When antibiotic treatment is begun within 1 hour after 550 r whole body radiation, survival is the rule. Delay in starting treatment is critical, for the longer the delay, the higher the mortality, even though the stool cultures meanwhile become coliform-free.

When antibiotic is started prior to or immediately after radiation exposure, it must be continued for at least 3 weeks if maximum effectiveness is to be obtained. The shorter the postradiation period of treatment, the greater the mortality. This suggests that the defense systems involved require protection for at least 3 weeks in order to permit return of maximal function.

The non-absorbable intestinal antibiotics are effective only when cultural data demonstrate successful elimination of the coliform flora in the gut.

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