The sequences of the RNA genomes of the MMTVs from C3H and RIII mice that are transmitted via a non-germ line mechanism have been isolated. These sequences are present as provirus in the DNAs of mammary tumors of C3H, RIII, and GR mice but are not detected in mammary tumors of C3HfC57BL or BALB/c mice. These sequences are also not detected in the DNA of apparently normal tissues of several strains of mice. Independent polypeptide initiation sites have been demonstrated for the MMTV nonglycosylated proteins (p28, p22, p18, p14, and p10). This is further evidence for the lack of strict coordinate synthesis of these proteins in MMTV-infected cells. MMTVs from three different strains have been employed to establish productive infections in mink and feline cells. These virions contain all the RNA sequences as do murine grown MMTVs. Competitive molecular hybridization studies with MPV 70S RNA demonstrate that MPV and morphologically similar isolates constitute a distinct class of retroviruses. Approximately 20% of the MPV genome is endogenous to rhesus monkeys. This same conserved part of the MPV genome is also endogenous to all other species of old world monkeys tested. BIBLIOGRAPHIC REFERENCES: Schlom, J., Colcher, D., Drohan, W. and Schochetman, G.: Nucleic acid and protein studies of Mason-Pfizer virus. In Clemmensen, J., Yohn, D.S. (Eds.): Comparative Leukemia Research. Karger, Basel, 1976, pp. 134-140. Drohan, W., Kettmann, R., Colcher, D. and Schlom, J.: Isolation of the mouse mammary tumor virus sequences not transmitted as germinal provirus in the C3H and RIII mouse strains. J. of Virology 21: 986-995, 1977.