A newly isolated, highly potent murine leukemogenic virus induced a dual type of disease in 90 to 100 percent of mice of different strains. The disease was characterized by rapid and extreme proliferation of predominantly erythrocytic and leukocytic elements as early as 7 days postparenteral inoculation. Within 15 days after inoculation of newborn or adult mice, 50 to 100 percent developed palpable spleens. Fifty percent of the animals, with spleens weighing 30 to 50 times those of normal mice, died within 25 to 35 days. In mice that survived, the erythrocytopoiesis was followed by the development of lymphocytic leukemia beginning 30 to 45 days after inoculation. Leukemic tissues transplanted with ease and killed 100 percent of the recipients. An intense viremia occurred in infected mice at least as early as 16 days after inoculation. The virus extracted from plasma or from leukemic tissues was of such potency that a 10⁻⁷ dilution of the test material induced the disease in 50 percent of the recipients within 95 days. Virus particles measuring 100 mμ were seen in thin sections of plasma pellets and in malignant lymphoblasts from spleen. Osborne-Mendel rats, 8 different inbred strains of mice, and random-bred Swiss mice, were all highly susceptible. The virus was stable to lyophilization, −70° C, formalin, and was not neutralized by antiserum prepared against the murine leukemia viruses of Friend, Moloney, or Schoolman-Schwartz. The ratio of mice responding with erythrocytopoiesis only to those developing erythrocytopoiesis plus lymphoid leukemia was influenced by the infecting dose of virus and by the strain and age of the recipients.