Radioimmunoassays (RIAs) for the major external and internal proteins of mouse mammary tumor viruses (MMTVs) demonstrated that both molecules contain type specific as well as group specific reactivities. Polypeptide map analyses of individual MMTV proteins sustantiate these findings. The establishment of heterologous cells producing MMTV has led to the development of a rapid in-vitro assay for the replication of MMTV. A new type-B retrovirus has been isolated from Mus cervicolor that shares some antigenic reactivity with known MMTVs. The sequences of the RNA genome of the MMTVs for C3H and RIII mice that are transmitted via a non-germ line mechanism have been isolated and shown to be present in the DNA of mammary tumors that arise early in life, but absent in mammary tumors of low or moderate incidence strains.