RF mice with primary and passaged radiation-induced myeloid leukemia were examined with the electron microscope for the presence of virus particles. The spleen, liver, and bone marrow of nearly all such mice examined contained small numbers of these particles. In one of two animals in which other tissues (thymus, pancreas, salivary glands, lymph nodes, omentum, lung, and brain) were examined, the thymus and pancreas also showed small numbers of particles. Much larger numbers of particles of the same morphology were seen in tissue cultures derived from the serially passaged leukemias. One control tissue culture, but none of the control tissues, contained similar particles. The particles resembled those associated with AKR mouse lymphoid leukemia (Type C in Bernhard's morphological classification of tumor viruses). Although cell-free filtrates of the particle-containing tissues have shown lymphoid and myeloid leukemogenic activity, the possible etiologic significance of the particles remains to be established.