Murine leukemia virus, isolated from Ha/ICR Swiss mice during the course of experiments which originated with assay of human leukemias and tumors for the presence of oncogenic agents, was recovered within 1–2 weeks from organs of mice injected at birth with cell-free filtrates from leukemic tissues. The incidences of leukemia induced by filtrates made from organs of mice at weekly intervals from 2 to 13 weeks after injection at birth were comparable to those obtained with filtrate from organs of leukemic mice. There was about 1 log greater infectivity in filtrates from pooled kidney-liver-spleen than from brain, as measured by incidence of leukemia in injected mice. Dose-response curves showed a decrease in incidence and increase in latency of leukemia development with increasing dilutions of filtrates. Similar growth and dose-response curves were obtained in a number of experiments. Virus could not be recovered from weanling mice, which are resistant to induction of leukemia by virus, until 8 weeks after infection. It is suggested that weanling mice may develop an early immunity to virus infection which may be lost in time. Recovery of virus from Sprague-Dawley rats 2 weeks after injection at birth was poor; virus could not be recovered at 5 and 8 weeks of age. However, virus which was highly infectious, as determined by high incidence of leukemia in mice, could be recovered from rat leukemias induced by the mouse virus.


This work was supported in part by grants from the John A. Hartford Foundation and the USPHS.

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