Purification of viruses of C58 mouse leukemias was carried out by the following procedures; filtration, differential ultracentrifugation and Gessler's fluorocarbon extraction. The degree of virus purification was examined by fluorescent antibody method, electron microscopy, nucleic acid analysis and bioassay. The following results were obtained. 1. Large amount of host cell antigens was noted in virus suspension separated by Berkefeld-N and Chamberland L3 filtration and by differential ultracentrifugation. Therefore it was improper to perform immunofluorescent study because this procedure was concerned with examination of viral antigens. 2. Immunofluorescent procedure by Coons' direct method using anti-viral serum against fluarocarbon extracts showed partially granular or partially diffuse specific fluorescence in leukemic cells and intercellular spaces of the leukemic organs. 3. Electron microscopic observation revealed many virus-like particles in the fluorocarbon extracts. And 50% of animals inoculated with developed leukemias in these extracts. 4. Ultraviolet spectrophotometry of the fluorocarbon extracts demonstrated a maximal absorption at 260 mμ and a minimal absorption at 230 mμ with a Bechman spectrophotometer. It was clear from these data that large amount of nucleic acid was contained in these extracts. And also it was indicated by Bial's test that the fluorocarbon extracts of the leukemia organs contained large amount of RNA. 5. From above findings, fluorocarbon extraction appeared a superior procedure for fluorescent antibody technic in comparison with the other procedurs used. It was thought that the specific fluorescence in the immunofluorescent study represented viral antigen itself.