Monoclonal antibodies have been used to define the diversity of mammary tumor viral-coded gene products associated with murine mammary tumors. Monoclonal antibodies have been generated against disrupted mammary tumor viruses (MTVs) isolated from M. musculus, M. cervicolor and M. cookii. Antibodies directed against the gp36 external glycoprotein and p28 internal protein of these viruses demonstrated the presence of multiple epitopes that represent type, group and interspecies determinants. The monoclonals have also been used to distinguish six MTV isolates from each other, including both endogenous and exogenous viruses from the same strain, and a new virus isolate from BALB/c mice. Employing the immunoperoxidase technique, the monoclonal antibodies have been used to define a heterogeneity of expression of MTV gene products in primary mammary tumors of three Mus species. These studies have revealed that a given tumor-associated antigenic determinant may be expressed differentially in mammary tumors of two different species, among mammary tumors of the same species and, at times, in different areas of the same mammary tumor.