Preliminary results of an electron microscope study of a leukemia of the mouse transmissible by cell-free filtrates are reported. In twelve out of forty-seven specimens examined, virus-like particles were observed. So far, these particles have not been observed in control, non-leukemic animals. They are located inside the cytoplasm of leukemic cells which infiltrate the spleen or the liver. Their diameter is approximately 78 mµ. Their resemblance to other particles recently described in different mouse tumors is stressed. Their significance and, in correlation with ultrafiltration data, their possible etiological meaning are discussed.

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