Phytohemagglutinin-stimulated lymphocyte cultures of 75 cattle were examined by electron microscopy. Virus-like particles were observed in cultures from 5 of 9 cattle with lymphosarcoma. Particles of the same type were also in cultures from 15 of 24 cattle in a multiple-case herd, 15 of 26 cattle that had been inoculated with lymphosarcoma materials, and 2 of 16 aged cows from herds with no history of lymphosarcoma. The virus-like particles were primarily extracellular and characterized by an electron-dense nucleoid 60–90 mμ in diameter that was centrally located inside a limiting membrane. The diameter of complete particles was 90–120 mμ. Some particles had an intermediate membrane in close apposition to the nucleoid. Structures resembling budding particles were rare. The morphology and size of the particles were similar to those of the mature form of C-type viruses which are regarded as the cause of leukemia in other species.