The proximal tibial metaphysis of apparently healthy strain 101 mice, 3-4 weeks old, and (C3H X 101)F1 hybrids, 3-48 weeks old, was studied by electron microscopy. Budding, immature, and mature (C-type virus particles were found within trabecular bone tissue of 3 of 8 strain 101 and 4 of 12 (C3H X 101)F1 mice. The particles were most common in lacunae of aging osteocytes and were only occasionally associated with osteoblasts. Although the morphology of budding and immature particles appeared to be identical with that of typical C-type viruses, most of the mature forms of particles showed atypical structure and size. The electron-dense core was very large and not clearly defined, measuring approximately 70-130 nm in diameter. This diffuse core sometimes completely filled the space within the envelope of the particles. The diameters of the pleomorphic mature C-type particles ranged from approximately 90 to 150 nm. The possible association between the production of pleomorphic C-type virus particles by bone cells and spontaneous osteomagenesis in 101 and (C3H X 101)F1 mice is discussed.