Further electron microscope observations of tissues from Swiss and DBA/2 mice with leukemia transmissible by cell-free filtrates to the adult animals are presented. The cytological characteristics of the leukemic cells, the fine structure of the viruses, and the virus host-cell relationships have been examined. The leukemia virus has an external diameter averaging 870 A, and appears to be formed at the level of cell membranes by a budding process. The viruses are observed most frequently in intercellular spaces, but are also often found within cytoplasmic vacuoles of megakaryocytes. Lead hydroxide staining was applied to the study of the leukemic material. The viruses have been found to have a considerable affinity for this lead salt, only comparable in intensity to the affinity shown by RNP granules for the same chemical.

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