With lymphoid tumors (lymphocytic leukemia and lymphocytic leukosarcomatosis) induced in the mice of uncertain origin by smearing with 20-methylcholanthrene, the author tried to clarify the relationship between lymphocytic leukemia and lymphocytic leukosarcomatosis by performing bone-marrow and lymph-node tissue cultures in combination with cytological examinations, and obtained the following results. (1) Lymphocytic leukosarcomatosis is mostly aleukemic, and the percentage of lymphoblasts in the peripheral blood is low. Some cases of lymphocytic leukosarcomatosis reveal a marked increase of large lymphocytes in the peripheral blood. (2) The lymph nodes and bone marrow of aleukemic lymphocytic leukosarcomatosis consist mostly of lymphosarcoma cells with the greatest diameters of more than 15.3μ and marked lobulation of the nucleus. These cells are thought to be derived from Amano's "lymphogonia". In addition, in the lymph nodes and bone marrow of aleukemic lymphocytic leukosarcomatosis, there can also be found little atypical lymphoblasts with the greatest diameters of less than 15.3μ, what may be considered as leukemic cells. and it seems that those lymphoblasts found in the peripheral blood of leukosarcomatosis are derived from these leukemic cells. (3) The lymph nodes and bone marrow of aleukemic lymphocytic leukosarcomatosis are consisted mostly of lymphosarcoma cells, while, in leukemic lymphocytic leukosarcomatosis and lymphocytic leukemia, the percentage of leukemic cells increases and the percentage of lymphosarcoma cells decreases. (4) In the bone-marrow and lymph-node tissue cultures, aleukemic lymphosarcomatosis generally presents the growth pattern resembling tumor, while leukemic lymphocytic leukosarcomatosis shows the growth pattern exactly similar to that of acute lymphocytic leukemia. (5) In autopsy findings, lymphocytic leukosarcomatosis shows more remarkable enlargement of lymph nodes than lymphocytic leukemia. Histologically, in aleukemic lymphocytic leukosarcomatosis there is little tendency of lymphoblast infiltration and metastasis to the liver and kidneys. (6) Since aleukemic and leukemic leukosarcomatosis can transform progressively to lymphocytic leukemia, all these are believed to be essentially the same disease.