Electron microscope study of thymuses of both conventional and germ-free mice has revealed the presence of typical virus particles associated with the thymic lymphocytes or with the thymic epithelial cells. The particles resemble those associated with several murine leukemias and their viral nature seems convincingly substantiated by morphological observation. Germ-free mice are therefore not virus-free. The biological significance of these particles is still unknown and we can only speculate as to the possible relationship of these particles to the incidence of "spontaneous" leukemia, to the lymphocytosis stimulating factor of Metcalf, and to the numerous latent viral infections of laboratory mice.

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