In the authors' opinion, there are no microscopic or ultramicroscopic character istics that permit separation of cultured human lymphoblastoid cells derived from normal individuals and from patients with leukemia, Burkitt's lymphoma and infectious mononucleosis. Further, there seemed to be no difference between lymphoblasts stimulated with phytomitogens and those of established cell lines. The spectrum of cell forms in each cell line under various culture conditions included stem cells, lymphoblasts, lymphocytes, plasmablasts and macrophages. There were no fibroblasts in the authors' cultures. All of the more than one hundred cell lines that have been tested produced immunoglobulins or immunoglobulin fragments. No granulocytic or erythroblastic cells were seen in long‐term cultures. A majority of the cell lines have retained normal karyotypes. They have continued to multiply rapidly for months and years, and thus appear to have an infinite life.

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Experiments were carried out in 9 cases of AKR and C58-mouse leukemia and in 5 cases of C3H-, C58-, and dd-mouse mammary cancer in order to make the cause and mechanism of leukemo and carcino-genesis from the view point of cell genetics and virus. The results obtained from the present studies lead to the following conclusion.1) From the results based on our chromosomal studies in the mouse leukemias and mammary cancers, it is considered that even neoplastic cells are possessed of a difinite chromosomal arrangement and they are divided by regular mitosis and also that the fundamental change in malignant cells would be existent in the genetic component of neoplastic cells.2) The virus particles were detected in the neoplastic tissue of all the cases of spontaneous leukemias and mammary cancers and even in that of the X-ray induced leukemia, and viral origin of leukemo-genesis are shown by the results of transplantation with cell-free leukemic extracts. The phenomenon of the disappearance of virus from the leukemic tissue of long term transplanted leukemia and that of virus induction under the influence of X-ray irradiation suggest that the virus is directly linked with the chromosome as a provirus and take a direct part in the nuclear division of neoplastic cell.3) Nuclear involvement of virus was found in the leukemic cells and in the mammary cancer cells in mitosis or in resting stage.4) The origin of the tumor virus in these leukemias and mammary cancers were investigated and it was found that viruses were existent even in the mammary gland of healthy C314-mice and in the thymus tissue of healthy AKR -mice.