1) In vivo studies using cloned genetically defined mouse leukemia retroviruses administered to newborn mice of know backgrounds revealed significant differences in behavior of these agents, depending on the immunocompetence and the particular breed of the host. These biological differences include virus replication in the recipient, tumorigenicity, and tumor type induced. The specified virus-host interaction is consistent for each agent, whether it is an intact leukovirus prototype or a laboratory recombinant constructed in LCMB laboratories. Many tumor types induced by the various retroviruses have been deep-frozen for future viral, antigenic and genomic studies. 2) In vitro transformation experiments using oncoviruses (with and without promoters) to infect normal primate cultures continue to produce greatly altered morphologic and growth patterns; no fully malignant transformants have been proven to date. 3) Human and animal xenografts on athymic nude mice have been used to study tumor progression and malignant transformation.