A noninfectious hamster C-type oncornavirus (D9) associated with a spontaneous hamster lymphoma was characterized. The defective virions contained 70S RNA and the base composition was similar to that of a murine sarcoma-leukemia virus. With both endogenous and added templates, the virions were found to be deficient in DNA polymerase activity. These results suggest that DNA polymerase may be required for D9 infectivity.