The expression of mouse mammary tumor virus (MMTV) gene products was examined in several different murine mammary tissues which showed no production of complete extracellular MMTV B-particles. Analyses by specific competition radioimmunoassay demonstrated that these mammary tissues contained large amounts of protein immunologically related to, or identical to, the 28,000-dalton major nonglycosylated MMTV core protein (p28), but no immunologically detectable 52,000-dalton 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, and its possible significance for MMTV-mediated oncogenesis.