Mature and immature C-type particles were observed in the extracellular spaces and in vesicles as well as budding from tumor cells and cell cultures derived from a transplanted X-irradiation induced rat mammary tumor. The C-type particles were found at a frequency of 1.13 per cell section examined. The buoyant density of the C-type particles ranged from 1.088 to 1.127 g/cm3 in sucrose density gradient. Deposits of glycogen were frequently observed in the cytoplasm of the tumor cells in culture. A smaller particle, having a diameter of approximately 50 mm was also observed in these cell cultures. This latter particle was restricted to intracytoplasmic locations and was observed singularly as well as in groups. It was spherical in shape having an electron lucent center. The outer membrane of the particle was poorly defined simulating a ‘toothed wheel’. These smaller particles were observed in high concentration at 1.204 g/cm3 in a sucrose density gradient. Morphologically, the smaller particles resembled pinocytotic or coated vesic