The data presented in this paper define the interrelationship between two endogenous diseases in the same host. The Haas strain of germfree mice with congenitally acquired persistent lymphocytic choriomeningitis (LCM) virus infection developed the X-ray-induced lymphatic leukemia independent of LCM viremia. The immunoproliferative LCM-related lesions appeared after age 7 months, and the radiogenic lymphatic leukemia appeared earlier or during the same age period. Both diseases evolved with

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... nctive lesions at the appropriate time, with no evidence of interference nor of synergism between the two syndromes. The Haas strain of mice showed an unusual sensitivity to X rays which was associated with depletion of blood components and coincident death. The sensitivity of the mice to X rays might be a characteristic of the Haas mouse or the result of an LCM virus-induced alteration in hematopoiesis.