A virus that induces a generalized lymphocytic leukemia in mice was recovered from Sarcoma 37. Virus-induced leukemic cells, freely transplantable within a given strain, caused death of 100 percent of the recipients. Cell-free suspensions of high tumor-producing activity were prepared from Sarcoma 37, from virus-induced leukemias, and from whole-cell-induced leukemias by differential centrifugation alone or in combination with filtration. The virus was stable to freezing and lyophilization. Infection by such virus preparations was not limited to mice of a single genetic type, nor to the very young animal. The incidence of virus-induced leukemia was essentially 100 percent in 7 of 8 inbred strains as well as in random-bred Swiss mice. Equally high incidence values were obtained with 5 hybrid groups of mice and with weanling and adult BALB/c mice. After 10 selective virus passages in newborn BALB/c mice, the potency of the leukemia virus was significantly increased. This was evidenced by a decrease in time to leukemia development from greater than 6 months to 2.5–3.0 months for a 100 percent incidence. The histologic picture was similar in all the mice examined regardless of whether the disease was virus-induced or followed inoculation of neoplastic cells. No variation was detected in mice of different strains or age groups and no neoplasms other than those of the lymphocytic type were observed either grossly or microscopically in any of the test animals.