The target site and nature of the long-term residual lesion (LTRL) caused by exposure to ionizing radiations are not well understood. Monkeys (Macaca mulatta) were given 100-rad conditioning doses of γ-rays at 56-day intervals to accumulate 2400 rad. Eighty-four days after the 24th conditioning exposure, the monkeys were challenged to death with continuous low-dose-rate γ-rays. The response of hematopoietic tissue was observed as the primary site of acute and long-term radiation injury. Hemograms were performed on monkeys during the fractionation radiation regimen used to administer conditioning doses and during the subsequent continuously delivered challenge exposures. Recovery rate from radiation injury was found to be independent of total accumulated γ-ray dose. The LTRL from γ-ray conditioning doses totaling 2400 rad did not significantly decrease mean survival time (MST) in a radiation field of 24 rad/23.5-hr day.

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