Residual radiation injury (the depression in resistance to 140 R/day, 5 days a week) owed to 500 R given 30 days earlier has been measured in C57BL/6J female mice between the ages of 3 and 23 months. Control mice (previously unirradiated) demonstrate a parabolic relationship between age and resistance to this exposure regimen, whereas preirradiated (500 R) mice demonstrate resistance levels which are inversely related to age. The state of the bone marrow, in terms of endogenous colony-forming unit patterns, in previously unirradiated aging mice correlates with their resistance to fractionated exposures, but it does not predict the inverse age-resistance relationship in aging mice given 500 R 30 days earlier. Bone marrow transplantation immediately after a lethal dose of x-rays reduces the residual injury detectable in young adult mice which survive for 30 days but does not remove the age-dependence for residual injury induction. Shielding of the trunk during exposure allows the residual injury measured 30 days later to parallel the state of the bone marrow. We conclude, therefore, that the basis of the age-dependence for residual injury induction is not hematopoietic and propose that the basis is the increasing importance of gastrointestinal factors as the age of the mouse increases. In support of this interpretation is the linear decline in the gastrointestinal resistance and recovery potential as a function of age.

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