After intravenous (IV) injection with factor-dependent FDC-P1 cells, irradiated DBA/2 and BALB/c mice developed transplantable leukemias owing to neoplastic transformation of the injected cells in vivo. Increasing the radiation dose shortened the preleukemic latent period, and in female mice the frequency of leukemia development was higher and the latent period shorter than in male mice. In the preleukemic period, the injected FDC-P1 cells rapidly increased in number in hematopoietic organs of irradiated animals, reaching peak levels 3 to 5 weeks after injection; factor-independent transformed cells were not detected before day 45. In unirradiated animals, these events were delayed by several weeks, and long-term survivors did not harbor detectable FDC-P1 cells. FDC-P1 cells sampled from preleukemic mice frequently showed atypical colony formation and reduced cloning efficiency in vitro, suggesting the occurrence of a distinct preleukemic change. U16.6 cells produced leukemia only in irradiated recipients, and the leukemic cells usually remained factor dependent. The two contrasting models should be of value in further analyzing the mechanisms underlying radiation- induced leukemias.

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