The purpose of these studies is to define mouse mammary tumor virus (MMTV) gene expression as a function of mammary tumorigenesis in low and moderate incidence mouse strains. Analyses by specific competition radioimmunoassay demonstrated that many of these mammary tissues contained large amounts of the major MMTV core protein (p28), but no detectable MMTV envelope glycoprotein (gp52). Such noncoordinate MMTV genome expression was found in primary and transplanted mammary tumors, and in preneoplastic mammary tissues of BALB/c mice, as well as in normal mammary tissue of Swiss albino mice. These mammary tissues exemplify a naturally occurring animal model system for studying noncoordinate expression of MMTV gene products. A new MMTV has also been isolated from mice of a sublime of the BALB/c mouse strain. Monoclonal antibodies directed against MMTV proteins were used to clearly distinguish MMTV(BALB/c) from all other known MMTVs. These immunological and other molecular studies thus define MMTV(BALB/c) as a novel MMTV variant.