Groups of Sprague-Dawley, Long-Evans, and Collip rats were exposed at 5-10 weeks of age to repeated high doses of X-radiation alone, urethane alone, or X-radiation plus urethane at the same time. Cumulative mortality and tumor incidence were measured until the animals reached 17 months of age. The final cumulative incidence of mammary tumors was highest in the Sprague-Dawley rats and lowest in the Collip rats, while the final cumulative incidence of skin tumors and fatal lymphatic disorders showed the opposite trend. X-Radiation and urethane together produced more lymphatic disorders but fewer skin and mammary tumors than expected from the sum of the effects of the two agents separately in Collip rats. However, these results were not always paralleled by those obtained in the other two strains of rats. As judged from the cumulative mortality curves for all three strains of rats tested, it would appear that the overall carcinogenic stimulus and life-shortening effect of X-radiation and urethane together was not greater than the sum of their separate effects at the dosage levels studied.

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