An electron microscopic study of the H-1 virus, derived originally from a transplantable human tumor, shows that the virus consists of a dense core approximately 150 A in diameter. A coat 75 A wide, not made visible by the techniques used, may encircle the core, which makes the diameter of the “complete” virus 300 A. Virus particles have been observed enclosed in gray bodies in the cytoplasm of the Küpffer cells, in the interstitial cells of the kidney from 3- to 11-day-old infected hamsters, and in the chondrocytes of embryonic hamster cells incubated in media inoculated with the virus. Changes in the mitochondrial structure suggest that these organelles are intimately involved in a virus-cell interaction.