1. The phagocytic index of the healthy white rabbit can be increased significantly by x-rays. This effect is most marked when treatment is delivered at 140 KV, and when a 100 r dose is used. The maximal increase occurs at 48 hours after treatment.

  2. Repetition of the optimal dose produces an increase in the phagocytic index which is only moderately higher than that obtained with a single dose, but the index may be maintained at a high level for a short time. There is a definite tolerance of the animals to x-rays for producing an increase in the phagocytic index, beyond which point a depression occurs.

  3. Previously radiated animals show a return of their phagocytic indices to normal at varying lengths of time following treatment, so that subsequent treatments following this event again institute a rise in the phagocytic index which is identical with that observed in untreated animals.

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