1. Cytosine arabinoside induced objective, but temporary, decrease of tumor masses in three patients with lymphosarcoma and slight decrease in some lesions in two out of ten treated patients with disseminated carcinomatosis.

2. In doses of 3 to 50 mg./Kg. given at varying intervals, cytosine arabinoside induced definite megaloblastic changes in the marrow of all patients studied. Mitotic abnormalities similar to those found in other megaloblastic anemias also occurred.

3. Associated with bone marrow changes, depressions of hemoglobin, white blood cells and platelets in the peripheral blood were observed.

4. The exact mechanism of action of cytosine arabinoside has not been elucidated. It is speculated that because of the close structural similarity between cytidylic acid, cytosine arabinoside could interfere with DNA synthesis.

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