Repeated X-irradiation (4 × 450 rads) of mice produces a decreased rate of bone marrow regeneration to a suboptimal level, compared to that after a single dose of 450 rads. 14 MeV neutrons are more effective in producing this reduced level by a factor of 1.7-1.8. The irradiated host (single 850 rads or 4 × 450 rads X-rays) is responsible for a lower growth rate of colony-forming units. The suboptimal plateau however is a "cellular effect," and these are additive during endogenous repopulation. A host effect is not demonstrable in terms of changes in the colony-stimulating activity of the femur shaft in bone marrow cultures.

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