The response of bone marrow stem cells to repeated large doses of x-rays was investigated in the mouse using the spleen colony assay technique. Four repeated doses of 450 rad, separated by 24 days, produced residual injury in the system characterized by a reduction in the stem cell compartment size, and a difference in the recovery patterns after the last dose compared to the first. The reduced recovery rates are shown to be due to damage to the milieu rather than damage to the cells themselves.

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