Specificity of anti-Rauscher leukemia virus goat serum was strictly checked by purification, labelling with fluorescein and elimination of nonspecific reaction. Using this antiserum, antigen analyses of the spleen and liver cells of Rauscher leukemic mice and human leukemic cells were examined by the direct immunofluorescence technique and following results were obtained.By electron microscopy, the eclipse phase was reported to be about 1 week after virus inoculation, but it was found that viral antigen was observable already in three days after virus infection. In the experiment with human leukemias, cells from bone marrow or peripheral bloodsmears from patients with 20 acute leukemias (14 of myelocytic, 6 of lymphocytic), 6 chronic leukemias (3 of myelocytic, 3 of lymphocytic), 10 malignant lymphomas (4 of Hodgkin disease, 4 of lymphosarcoma, 2 of reticulum cell sarcoma), 2 multiple myeloma and 10 other disorders were tested with anti-Rauscher leukemia virus goat serum. However, specific fluorescence was not seen in any cells tested. Based on these findings, the role of virus in human leukemia was discussed.