Fromalin-treated extract of spleens of BALB/c mice infected with Rauscher leukemia was, used for protection and immunotherapy against Rauscher leukemia. In the group which was vaccinated at the same time with and one week after virus inoculation leukemia was not protected, although mean survival was more prolonged than in control group. In the group which was injected vaccine 6 weeks after virus inoculation 10 percent of mice were induced to remission. Furthermore, in the group which was vaccinated at 3 weeks after virus inoculation, likewise 10 percent of mice were completely protected against leukemia without manifesting any leukemic sign. In leukemic mice peripheral bolod pictures were characterized by marked increase of nucleated cells and presence of erythroblasts and Rauscher cells. Enormous hepatosplenomegaly was due to proliferating reticulum cells and erythroblasts. In mice which were protected against leukemia and induced to remission with vaccines, no abnormal findings were present in either peripheral blood pictures or histological sections of the spleen and liver. With electron microscopy numerous virus particles were observed in the spleen of leukemic mice, but not observed in the spleen of mice which were protected against leukemia and induced to remission.