Electron microscopy of the spleen and bone marrow from a cat with spontaneous feline lymphosarcoma revealed leukemogenic-type viruses. Newborn kittens and adult cats were inoculated with an extract or a filtrate prepared from tumor tissue from the cat. Infected kittens and cats were bled by cardiac puncture approximately 4 and 6 weeks after injection, and pellets of blood platelets were prepared for electron microscopy. The same type of virus as in the donor animal budded from the surface of the platelets and into platelet vacuoles, and complete virus particles were found free between the platelets and in platelet vacuoles. Platelet pellets from uninfected kittens bled at 28 and 42 days showed no virus particles. Bone marrow, thymus, and spleen from each of 2 kittens killed 28 days after inoculation contained fully formed and budding particles of the leukemogenic type. The clinical disease developed in some kittens.