The close immunologic relation between the group-specific and polymerase proteins of the wild mouse derived and the established strains of mouse type C oncornaviruses, and the lack of any unusual structural polypeptide or RNA components in the wild mouse virions, eliminate the possibility of detectable contamination by a class of virus different from the mouse oncornavirus class in the wild mouse virus preparations. Liquid hybridization studies with a wild mouse viral 70S 3H-RNA and cellular DNA under conditions of DNA excess, suggest that a significant fraction but not all of the virus-specific nucleotide sequences is present in all normal and tumored wild mouse tissues tested. These virus related sequences may possibly be attributed to a hypothetical endogenous inherited type C virus genome(s) carried by all wild mice or to an infection by one or more but not all of the different exogenous strains of wild mouse type C viruses which could possibly be present in the virus preparation used. The findings are consistent with wild mouse derived type C viruses being either entirely exogenous or a mixture of endogenous and exogenous viruses. This interpretation is also consistent with the earlier observation that certain wild mouse type C viruses are exogenously transmitted, transplacentally, and via milk. The possible relation of the virus heterogeneity or the distinct characteristics of the virus surface molecules to the diverse pathogenicity of the wild mouse oncornaviruses is discussed.