Cells from an experimental sarcoma produced by inoculation of BHK 21 cells clone 13 cells in hamsters showed many similar ultrastructural features with the original cultured cells. Clear and dark cells, probably representing different metabolic stages of the same tumor cell, were noted. In both tumor and culture cells, type R or H virus-like particles could be detected. The high number of R or H particles observed in tumor cells suggests their participation in the oncogenesis of this neoplasm. The tumor cells contained several dilated endoplasmic reticulum cisternae with round and filamentous R type virus particles. Many tumor cells presented nuclear bodies of the filamentous and granular types.