Life-shortening effects of whole-body X-Irradiation of C3H, BALB/c, RFM, C57BL/6,${\rm C}3{\rm CF}_{1}$,${\rm B}6{\rm RFMF}_{1}$, and a four-way cross were compared. Male and female mice exposed to 300 R at 5-6 weeks of age showed significant life shortening in all strains and crosses. Females were, in general, 1.5 times more sensitive than males of the same strain, suggesting a constant effect of sex. There were substantial differences in sensitivity within the same sex among strains and it was found that this strain-specific difference was highly correlated with body weight at 6 weeks of age. On this basis a hypothesis has been advanced which views some of the strain-specific differences in radiation sensitivity as due to differences in maturational rate or to processes which are highly correlated with maturational rate.

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